Surface hardening treatment device and surface hardening treatment method

ABSTRACT

The present, invention includes: an in-furnace atmospheric gas concentration detector configured to detect a hydrogen concentration or an ammonia concentration in a processing furnace; an in-furnace nitriding potential calculator configured to calculate a nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmospheric gas concentration detector; and a gas-introduction-amount controller configured to change an introduction amount of each of the plurality of furnace introduction gases except for the ammonia decomposition gas while keeping an introduction amount of the ammonia decomposition gas constant. based on the calculated nitriding potential in the processing furnace and a target nitriding potential, such that the nitriding potential in the processing furnace is brought close to the target nitriding potential.

TECHNICAL FIELD

The present invention relates to a surface hardening treatment deviceand a surface hardening treatment method which can perform a surfacehardening treatment, such as nitriding, nitrocarburizing, nitridingquenching (austenitic nitriding), and the like, for a work made ofmetal.

BACKGROUND ART

Among various surface hardening treatments for a work made of metal suchas steel, there is a strong need for nitriding because it is a lowdistortion treatment. As a specific nitriding method. there are a gasmethod, a salt bath method, a plasma method. and the like.

Among these methods, the gas method is comprehensively superior whenconsidering quality, environmental properties, mass productivity, andthe like. Carburizing, carbonitriding or induction hardening (quenching)involved in hardening a mechanical part causes distortion, but thedistortion can be improved when a nitriding treatment by a gas method(gas nitriding treatment) is used. A nitrocarburizing treatment by a gasmethod (gas nitrocarburizing treatment) involved in carburizing is alsoknown as a treatment of the same kind as the gas nitriding treatment.

The gas nitriding treatment is a process in which only nitrogen ispermeated and diffused into a work, in order to harden a surface of thework. In the gas nitriding treatment, an ammonia gas alone, a mixed gasof an ammonia gas and a nitrogen gas, a mixed gas of an ammonia gas andan ammonia decomposition gas (which consists of 75% hydrogen and 25%nitrogen, and is also called an AX gas), or a mixed gas of an ammoniagas an ammonia decomposition gas and a nitrogen gas, is introduced intoa processing furnace in order to perform a surface hardening treatment.

On the other hand, the gas nitrocarburizing treatment is a process inwhich carbon is secondarily permeated and diffused into a work togetherwith nitrogen, in order to harden a surface of the work. For example, inthe gas nitrocarburizing treatment, a mixed gas of an ammonia gas, anitrogen gas and a carbon dioxide gas (CO₂) or a mixed gas of an ammoniagas, a nitrogen gas, a carbon dioxide gas and a carbon monoxide gas (CO)is introduced into a processing furnace in order to perform a surfacehardening treatment, as a plurality of furnace introduction gases.

The basis of an atmosphere control in the gas nitriding treatment and inthe gas nitrocarburizing treatment is to control a nitriding potential(K_(N)) in a furnace. By controlling the nitriding potential (K_(N)), itis possible to control a volume fraction of the γ′ phase (Fe₄N) and theε phase (Fe₂₋₃N) in a compound layer generated on a surface of a steelmaterial and/or to achieve a process in which such a compound layer isnot generated. That is to say, it is possible to obtain a wide range ofnitriding qualities. For example, according to JP-A-2016-211069 (PatentDocument 1), the bending fatigue strength and/or the wear resistance ofa mechanical part may be improved by selecting the γ′ phase andincreasing its thickness, which can achieve a further high functionalityof the mechanical part.

On the other hand, the nitrocarburizing treatment is used for makingpositive use of the ε phase which is hard, for example in order toimprove the wear resistance (“Nitriding and Nitrocarburizing on IronMaterials”, second edition (2013), pages 81-86 (Dieter Liedtke et al.,Agune Technical Center) Non-Patent Document 1).

In the gas nitriding treatment and the gas nitrocarburizing treatment asdescribed above, in order to control an atmosphere in the processingfurnace in which the work is arranged, an in-furnace atmospheric gasconcentration measurement sensor configured to measure a hydrogenconcentration in the furnace or an ammonia concentration in the furnaceis installed. Then, the in-furnace nitriding potential is calculatedfrom the measured value of the in-furnace atmospheric gas concentrationmeasurement sensor, and is compared with a target (set) nitridingpotential, in order to control the flow rate of each furnaceintroduction gas (“Heat Treatment”, Volume 55, No. 1, pages 7-11(Yasushi Hiraoka. Yoichi Watanabe). Non-Patent Document 2). As for themethod of controlling each furnace introduction gas, a method ofcontrolling the total amount while keeping the flow rate ratio betweenthe respective furnace introduction gases constant is well known(“Nitriding and Nitrocarburizing on Iron Materials”, second edition(2013), pages 158-163 (Dieter Liedtke et al., Agune Technical Center):Non-Patent Document 3).

JP-B-5629436 (Patent Document 2) has disclosed a device which canperform both a first control step of controlling a total introductionamount of a plurality of furnace introduction gases while keeping a flowrate ratio between the plurality of furnace introduction gases constantand a second control step of controlling an introduction amount of eachof the plurality of furnace introduction gases while changing a flowrate ratio between the plurality of furnace introduction gases (eitherone of the first control step and the second control step is selectivelyperformed at a time) (JP-B-5629436: Patent Document 2). However,JP-B-5629436 (Patent Document 2) has disclosed only one example ofnitriding treatment in which the first control step is effective(paragraphs 0096 and 0099 of JP-B-5629436 (Patent Document 2): thenitriding potential 3.3 is precisely controlled by controlling the totalintroduction amount of the ammonia gas and the nitrogen gas whilekeeping the flow rate ratio of NH₃ (ammonia gas):N₂ (nitrogengas)=80:20″), but there is no description as to what kind of nitridingtreatment or nitrocarburizing treatment for which the second controlstep should be adopted. In addition. JP-B-5629436 (Patent Document 2)has disclosed no specific example of the second control step.

The method of controlling a total introduction amount of a plurality offurnace introduction gases while keeping a flow rate ratio between theplurality of furnace introduction gases constant is advantageous in thatthe total used amount of the plurality of furnace introduction gases maybe made smaller. However, it has been known that the controllable rangeof nitriding potential by means of this method is narrow. In order tocope with this problem, the present inventor has already developed acontrol method that can achieve a wide controllable range of nitridingpotential on the side of lower nitriding potential (for example, about0.05 to 1.3 at 580° C.) and has obtained JP-B-6345320 (Patent Document3). According to the control method disclosed in JP-B-6345320 (PatentDocument 3), an introduction amount of each of the plurality of furnaceintroduction gases is controlled by changing a flow rate ratio betweenthe plurality of furnace introduction gases while keeping a totalintroduction amount of the plurality of furnace introduction gasesconstant. such that the nitriding potential in the processing furnace isbrought close to the target nitriding potential.

(Fundamentals of the Gas Nitriding Treatment)

The fundamentals of the gas nitriding treatment are chemicallyexplained. In the gas nitriding treatment, in the processing furnace(gas nitriding furnace) in which the work is arranged, a nitridingreaction represented by the following formula (1) occurs.

NH₃→[N]+3/2 H₂  (1)

At this time, the nitriding potential K_(N) is defined by the followingformula (2).

K _(N) =P _(NH3) /P _(H2) ^(3/2)  (2)

Herein, the partial pressure of ammonia in the furnace is represented byP_(NH3), and the partial pressure of hydrogen in the furnace isrepresented by P_(H2). The nitriding potential K_(N) is well known as anindex representing the nitriding ability of the atmosphere in the gasnitriding furnace.

On the other hand, in the furnace during the gas nitriding treatment, apart of the ammonia gas introduced into the furnace is thermallydecomposed into a hydrogen gas and a nitrogen gas according to areaction represented by the following formula (3).

NH₃→½N₂+3/2H₂  (3)

In the furnace, the thermal decomposition reaction represented by theformula (3) mainly (dominantly) occurs, and the nitriding reactionrepresented by the formula (1) is almost negligible quantitatively.Therefore, if the in-furnace ammonia concentration consumed in thereaction represented by the formula (3) or the hydrogen gasconcentration generated in the reaction represented by the formula (3)is known, the nitriding potential can be calculated. That is to say,since 1.5 mol of hydrogen and 0.5 mol of nitrogen are generated from 1mol of ammonia, if the in-furnace ammonia concentration is measured, thein-furnace hydrogen concentration can also be known and thus thenitriding potential can be calculated. Alternatively, if the in-furnacehydrogen concentration is measured, the in-furnace ammonia concentrationcan also be known, and thus the nitriding potential can also becalculated.

The ammonia gas that has been introduced (flown) into the gas nitridingfurnace is circulated through the furnace and then discharged outsidethe furnace. That is to say. in the gas nitriding treatment, a fresh(new) ammonia gas is continuously flown into the furnace with respect tothe existing gases in the furnace. so that the existing gases arecontinuously discharged out of the furnace (extruded at the supplypressure).

Herein, if the flow rate of the ammonia gas introduced into the furnaceis small, the gas residence time thereof in the furnace becomes long, sothat the amount of the ammonia gas to be thermally decomposed increases,which increases the amount of the sum of the nitrogen gas and thehydrogen gas generated by the thermal decomposition reaction. On theother hand, if the flow rate of the ammonia gas introduced into thefurnace is large, the amount of the ammonia gas to be discharged outsidethe furnace without being thermally decomposed increases, whichdecreases the amount of the sum of the nitrogen gas and the hydrogen gasgenerated by the thermal decomposition reaction.

(Fundamentals of the Flow Rate Control)

Next, the fundamentals of the flow rate control are explained in thecase wherein an ammonia gas is used as a solo (single) furnaceintroduction gas. When the degree of thermal decomposition of theammonia gas introduced into the furnace is represented by s (0<s<1), thegas reaction in the furnace is represented by the following formula (4).

NH₃→(1−s)/(1+s)NH₃+0.5s/(1+s)N₂+1.5s/(1+s)H₂  (4)

Herein, the left side represents the furnace introduction gas (ammoniagas only), the right side represents the in-furnace atmospheric gases(gas composition) including a part of the ammonia gas remained withoutbeing thermally decomposed, and the nitrogen gas and the hydrogen gasgenerated in the ratio of 1:3 by the thermal decomposition of theammonia gas. Therefore, when the hydrogen concentration in the furnaceis measured by means of a hydrogen sensor, 1.5s/(1+s) on the right sidecorresponds to the measured value of the hydrogen sensor, and thus thedegree of the thermal decompositions of the ammonia gas introduced intothe furnace can be calculated from the measured value. Thereby, theammonia concentration in the furnace corresponding to (1−s)/(1+s) on theright side can also be calculated. That is to say. the in-furnacehydrogen concentration and the in-furnace ammonia concentration can beknown only from the measured value of the hydrogen sensor. Thus, thenitriding potential can be calculated.

Similarly, even when a plurality of furnace introduction gases are used,it is possible to control the nitriding potential K_(N). For example,when an ammonia gas and a nitrogen gas are used as two furnaceintroduction gases and the introduction ratio therebetween is x:y (bothx and y are known, and x+y=1. For example, if x=0.5, y=1-0.5=0.5(NH₃:N₂=1:1). the gas reaction in the furnace is represented by thefollowing formula (5).

xNH₃+(1−x)N₂ →x(1−s)/(1+sx)NH₃+(0.5sx+1−x)/(1+sx)N₂+1.5sx (1+sx)H₂  (5)

Herein, the right side represents the in-furnace atmospheric gases (gascomposition) including a part of the ammonia gas remained without beingthermally decomposed, the nitrogen gas and the hydrogen gas generated inthe ratio of 1:3 by the thermal decomposition of the ammonia gas, andthe nitrogen gas remained as introduced on the left side (without beingdecomposed in the furnace). Now, in the hydrogen concentration on theright side, i.e., 1.5sx/(1+sx). x is known (for example. x=0.5), andthus only the degree of the thermal decomposition s of the ammonia gasintroduced into the furnace is unknown. Therefore, in the same way as inthe formula (4), the degree of the thermal decomposition s of theammonia gas introduced into the furnace can be calculated from themeasured value of the hydrogen sensor. Thereby, the ammoniaconcentration in the furnace can also be calculated. Thus, the nitridingpotential can be calculated.

When the introduction ratio between the respective furnace introductiongases is not fixed, the in-furnace hydrogen concentration and thein-furnace ammonia concentration include two variables, i.e., the degreeof the thermal decompositions of the ammonia gas introduced into thefurnace and the introduction ratio x of the ammonia gas. In general, amass flow controller (MFC) is used as a device for controlling each gasflow rate. Thus, the introduction ratio x of the ammonia gas can becontinuously read out as a digital signal based on flow rate values ofthe respective gases. Therefore, the nitriding potential can becalculated based on the formula (5) by combining this introduction ratiox and the measured value of the hydrogen sensor.

On the other hand, the fundamentals of the gas nitrocarburizingtreatment are chemically explained. In the gas nitrocarburizingtreatment, in the processing furnace (gas nitrocarburizing furnace) inwhich the work is arranged, a carbon supply reaction represented by thefollowing formulas (6) and (7) occurs.

2CO→[C]+CO₂  (6)

CO+H₂→[C]+H₂O  (7)

As clearly seen from the formulas (6) and (7), the carbon supply sourceis a carbon monoxide gas. The carbon monoxide gas may be directlyintroduced into the processing furnace, or may be generated in theprocessing furnace from a carbon dioxide gas. Herein, in the processingfurnace, an equilibrium reaction represented by the following formula(8) is established.

CO₂+H₂→CO+H₂O  (8)

In addition, in the processing furnace, regarding H₂O, anotherequilibrium reaction represented by the following formula (9) isestablished.

2H₂O→O₂+2H₂  (9)

As seen from the above explanation, an amount of hydrogen (whose moleratio is represented by w) consumed by the reactions represented by theformulas (8) and (9) is correlated to an amount of oxygen in theprocessing furnace. Thus, it is preferable to obtain the degree of thethermal decomposition s of the ammonia gas after calculating the moleratio w based on a measured value of an oxygen sensor on the assumptionthat a measured value of the hydrogen sensor corresponds to(1.5sx−w)/(1+sx), rather than to directly substitute a measured value ofthe hydrogen sensor for 1.5sx/(1+sx) in the formula (5).

An equilibrium constant of the formula (9) is K=pH₂O/(pH₂·pO₂ ^(1.5)),wherein pH₂O, pH₂ and pO₂ are partial pressures of H₂O, H₂ and O₂ in theprocessing furnace, respectively. Thus, from an equilibrium constant Kknown correspondingly to an in-furnace temperature condition andmeasured values of both the oxygen sensor and the hydrogen sensor (=pH₂.pO₂), it is possible to calculate the value of pH₂O. Then, as clearlyseen from the formulas (8) and (9), the amount of hydrogen w consumed bythose reactions corresponds to the value of pH₂O. Therefore, it ispossible to obtain the value of w, and thus in turn it is possible toobtain the degree of the thermal decompositions of the ammonia gas.

Patent Document

The Patent Document 1 cited in the present specification isJP-A-2016-211069.

-   The Patent Document 2 cited in the present specification is    JP-B-5629436.-   The Patent Document 3 cited in the present specification is    JP-B-6345320.

Non-Patent Document

-   The Non-patent Document 1 cited in the present specification is    “Nitriding and Nitrocarburizing on Iron Materials”, second edition    (2013), pages 81-86 (Dieter Liedtke et al., Agune Technical Center).-   The Non-patent Document 2 cited in the present specification is    “Heat Treatment”, Volume 55, No. 1, pages 7-11 (Yasushi Hiraoka,    Yoichi Watanabe).-   The Non-patent Document 3 cited in the present specification is    “Nitriding and Nitrocarburizing on Iron Materials”, second edition    (2013), pages 158-163 (Dieter Liedtke et al., Agune Technical    Center).-   The Non-patent Document 4 cited in the present specification is    “Effect of Compound Layer Thickness Composed of γ′-Fe₄N on    Rotated-Bending Fatigue Strength in Gas-Nitrided JIS-SCM435 Steel”,    Materials Transactions. Vol. 58. No. 7 (2017), pages 993-999 (Y.    Hiraoka and A. Ishida).-   The Non-patent Document 5 cited in the present specification is “The    Special Steel”. Volume 61, No. 3. pages 17-19 (Hitoshi Kabasawa)

SUMMARY OF INVENTION Technical Problem

The present inventor has repeated diligent examination and variousexperiments about the gas nitrocarburizing treatment in which aplurality of furnace introduction gases including an ammonia gas and anammonia decomposition gas are introduced into a processing furnace. As aresult, the present inventor has found that a control of nitridingpotential which is sufficient for practical use can be achieved bychanging an introduction amount of each of the plurality of furnaceintroduction gases except for the ammonia decomposition gas whilekeeping an introduction amount of the ammonia decomposition gasconstant. as a control for bringing the nitriding potential in theprocessing furnace close to the target nitriding potential.

The present invention has been made based on the above findings. It isan object of the present invention to provide a surface hardeningtreatment device and a surface hardening treatment method which arecapable of achieving a control of nitriding potential which issufficient for practical use, in a gas nitrocarburizing treatment inwhich a plurality of furnace introduction gases including an ammonia gasand an ammonia decomposition gas are introduced into a processingfurnace.

Solution to Problem

The present invention is a surface hardening treatment device forperforming a gas nitrocarburizing treatment as a surface hardeningtreatment for a work arranged in a processing furnace by introducing aplurality of furnace introduction gases including an ammonia gas and anammonia decomposition gas, the surface hardening treatment deviceincluding: an in-furnace atmospheric gas concentration detectorconfigured to detect a hydrogen concentration or an ammoniaconcentration in the processing furnace; an in-furnace nitridingpotential calculator configured to calculate a nitriding potential inthe processing furnace based on the hydrogen concentration or theammonia concentration detected by the in-furnace atmospheric gasconcentration detector; and a gas-introduction-amount controllerconfigured to change an introduction amount of each of the plurality offurnace introduction gases except for the ammonia decomposition gaswhile keeping an introduction amount of the ammonia decomposition gasconstant, based on the nitriding potential in the processing furnacecalculated by the in-furnace nitriding potential calculator and a targetnitriding potential, such that the nitriding potential in the processingfurnace is brought close to the target nitriding potential.

According to the present invention, it has been confirmed that a controlof nitriding potential of a relatively wider range (in particular, acontrol of nitriding potential which is relatively lower) can beachieved by changing the introduction amount of each of the plurality offurnace introduction gases except for the ammonia decomposition gaswhile keeping the introduction amount of the ammonia decomposition gasconstant.

It is desirable that the introduction amount of the ammoniadecomposition gas, which is to be kept constant, has been predeterminedin advance by conducting a preliminary experiment before an actualoperation. This is because, in fact, the degree of the thermaldecomposition of the ammonia gas may be also influenced by in-furnaceenvironment of the furnace to be used or the like.

Preferably, the surface hardening treatment device of the presentinvention further includes: an in-furnace oxygen concentration detectorconfigured to detect an oxygen concentration in the processing furnace,wherein the in-furnace nitriding potential calculator is configured tocalculate the nitriding potential in the processing furnace based on thehydrogen concentration or the ammonia concentration detected by thein-furnace atmospheric gas concentration detector and the oxygenconcentration detected by the in-furnace oxygen concentration detector.

As described above, in the nitrocarburizing treatment, hydrogen isconsumed in a carbon supply reaction such that water (H₂O) is generated.The generated amount of the water (H₂O) establishes an equilibrium withthe amount of the oxygen in the processing furnace. Thus, a more precisenitriding potential can be achieved by detecting the oxygenconcentration in the processing furnace by the in-furnace oxygenconcentration detector and using the oxygen concentration forcalculating the nitriding potential in the processing furnace.

In addition, it is preferable that the gas-introduction-amountcontroller is configured to control the introduction amount C1, * * * ,CN (N is an integer of one or more) of each of the plurality of furnaceintroduction gases except for the ammonia gas and the ammoniadecomposition gas, using a factor of proportionality c1, * * * , cNassigned to each of the plurality of furnace introduction gases exceptfor the ammonia gas and the ammonia decomposition gas, such thatC1=c1×(A+x×B). * * * . cN=cN×(A+x×B) wherein the introduction amount ofthe ammonia gas is represented by A, the introduction amount of theammonia decomposition gas is represented by B, and a predeterminedconstant is represented by x.

According to the actual experiments conducted by the present inventor,it has been confirmed that, when the above control conditions areadopted, a control of nitriding potential of a relatively wider range(in particular, a control of nitriding potential which is relativelylower) can be achieved.

For example, the value of x is 0.5. This is because the amount ofhydrogen generated in the processing furnace by the thermaldecomposition of the ammonia gas of 1 mol is 1.5 mol while the amount ofhydrogen supplied from the ammonia decomposition gas of 1 mol into theprocessing furnace is 0.75 mol (¾ mol), and thus 1.5:0.75=1:0.5. Thevalue is explained as a factor for converting the introduction amount ofthe ammonia decomposition gas B into the introduction amount of theammonia gas A with regard to the amount of hydrogen.

However, the value of x does not have to be strictly 0.5. If the valueof x is roughly within a range of 0.4 to 0.6, a control of nitridingpotential which is sufficient for practical use can be achieved.

The plurality of furnace introduction gases includes a carbon dioxidegas as a carburizing gas. Alternatively, the plurality of furnaceintroduction gases includes a carbon monoxide gas as a carburizing gas.

Alternatively, the plurality of furnace introduction gases includes acarbon dioxide gas and a nitrogen gas, or includes a carbon monoxide gasand a nitrogen gas.

In addition, the present invention can be recognized as a surfacehardening treatment method. That is to say, the present invention is asurface hardening treatment method of performing a gas nitrocarburizingtreatment as a surface hardening treatment for a work arranged in aprocessing furnace by introducing a plurality of furnace introductiongases including an ammonia gas and an ammonia decomposition gas, thesurface hardening treatment method including: an in-furnace atmosphericgas concentration detecting step of detecting a hydrogen concentrationor an ammonia concentration in the processing furnace: an in-furnacenitriding potential calculating step of calculating a nitridingpotential in the processing furnace based on the hydrogen concentrationor the ammonia concentration detected at the in-furnace atmospheric gasconcentration detecting step:and a gas-introduction-amount controllingstep of changing an introduction amount of each of the plurality offurnace introduction gases except for the ammonia decomposition gaswhile keeping an introduction amount of the ammonia decomposition gasconstant, based on the nitriding potential in the processing furnacecalculated at the in-furnace nitriding potential calculating step and atarget nitriding potential, such that the nitriding potential in theprocessing furnace is brought close to the target nitriding potential.

In addition, the present invention is a surface hardening treatmentdevice for performing a gas nitrocarburizing treatment as a surfacehardening treatment for a work arranged in a processing furnace byintroducing a plurality of furnace introduction gases including anammonia gas, an ammonia decomposition gas and a carburizing gas, thesurface hardening treatment device including: an in-furnace atmosphericgas concentration detector configured to detect a hydrogen concentrationor an ammonia concentration in the processing furnace; an in-furnacenitriding potential calculator configured to calculate a nitridingpotential in the processing furnace based on the hydrogen concentrationor the ammonia concentration detected by the in-furnace atmospheric gasconcentration detector:and a gas-introduction-amount controllerconfigured to change an introduction amount of each of the ammonia gasand the carburizing gas while keeping an introduction amount of theammonia decomposition gas constant, based on the nitriding potential inthe processing furnace calculated by the in-furnace nitriding potentialcalculator and a target nitriding potential. such that the nitridingpotential in the processing furnace is brought close to the targetnitriding potential.

The feature of the above invention is to change the introduction amountof each of the ammonia gas and the carburizing gas while keeping theintroduction amount of the ammonia decomposition gas constant, while anintroduction amount of each of the rest of the plurality of furnaceintroduction gases is not conditioned. Accordingly, the scope of theabove invention can clearly cover any manner in which a certain minuteamount of gas (whose flow ratio is about 1% or less) is introduced to anextent that it does not substantially involve the reactions. Forexample, when two or more kinds of carburizing gases are introduced, theabove invention is applicable, in which an introduction amount of themain carburizing gas may be changed and an introduction amount of theother carburizing gas minutely introduced may be constant, according towhich a control of nitriding potential of a relatively wider range (inparticular, a control of nitriding potential which is relatively lower)can be achieved.

In this case, it is preferable that the gas-introduction-amountcontroller is configured to control the introduction amount C1 of thecarburizing gas, using a factor of proportionality c1 assigned to thecarburizing gas, such that C1=c1×(A+x×B), wherein the introductionamount of the ammonia gas is represented by A, the introduction amountof the ammonia decomposition gas is represented by B, and apredetermined constant is represented by x.

In addition, the present invention is a surface hardening treatmentdevice for performing a gas nitrocarburizing treatment as a surfacehardening treatment for a work arranged in a processing furnace byintroducing a plurality of furnace introduction gases including anammonia gas, an ammonia decomposition gas, a carburizing gas and anitrogen gas, the surface hardening treatment device including: anin-furnace atmospheric gas concentration detector configured to detect ahydrogen concentration or an ammonia concentration in the processingfurnace: an in-furnace nitriding potential calculator configured tocalculate a nitriding potential in the processing furnace based on thehydrogen concentration or the ammonia concentration detected by thein-furnace atmospheric gas concentration detector:and agas-introduction-amount controller configured to change an introductionamount of each of the ammonia gas, the carburizing gas and the nitrogengas while keeping an introduction amount of the ammonia decompositiongas constant, based on the nitriding potential in the processing furnacecalculated by the in-furnace nitriding potential calculator and a targetnitriding potential, such that the nitriding potential in the processingfurnace is brought close to the target nitriding potential.

The feature of the above invention is to change the introduction amountof each of the ammonia gas, the carburizing gas and the nitrogen gaswhile keeping the introduction amount of the ammonia decomposition gasconstant, while an introduction amount of each of the rest of theplurality of furnace introduction gases is not conditioned. Accordingly,the scope of the above invention can clearly cover any manner in which acertain minute amount of gas (whose flow ratio is about 1% or less) isintroduced to an extent that it does not substantially involve thereactions. For example, when two or more kinds of carburizing gases areintroduced, the above invention is applicable, in which an introductionamount of the main carburizing gas may be changed and an introductionamount of the other carburizing gas minutely introduced may be constant.according to which a control of nitriding potential of a relativelywider range (in particular, a control of nitriding potential which isrelatively lower) can be achieved.

In this case, it is preferable that the gas-introduction-amountcontroller is configured to control the introduction amount C1 of thecarburizing gas and the introduction amount C2 of the nitrogen gas,using a factor of proportionality c1 assigned to the carburizing gas anda factor of proportionality c2 assigned to the nitrogen gas, such thatC1=c1×(A+x×B) and C2=c2×(A+x×B), wherein the introduction amount of theammonia gas is represented by A, the introduction amount of the ammoniadecomposition gas is represented by B. and a predetermined constant isrepresented by x.

Effects of Invention

According to the present invention, it has been confirmed that a controlof nitriding potential of a relatively wider range (in particular, acontrol of nitriding potential which is relatively lower) can beachieved by changing the introduction amount of each of the plurality offurnace introduction gases except for the ammonia decomposition gaswhile keeping the introduction amount of the ammonia decomposition gasconstant.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing a surface hardening treatment deviceaccording to a first embodiment of the present invention:

FIG. 2 is a graph showing a control of furnace introduction gasesaccording to an example 1-1;

FIG. 3 is a graph showing a control of nitriding potential according tothe example 1-1:

FIG. 4 is a graph showing a control of furnace introduction gasesaccording to an example 1-3;

FIG. 5 is a graph showing a control of nitriding potential according tothe example 1-3;

FIG. 6 is a table comparing the examples 1-1 to 1-3 with theirrespective comparative examples;

FIG. 7 is a schematic view showing a surface hardening treatment deviceaccording to a second embodiment of the present invention;

FIG. 8 is a graph showing a control of furnace introduction gasesaccording to an example 2-2;

FIG. 9 is a graph showing a control of nitriding potential according tothe example 2-2;

FIG. 10 is a table comparing the examples 2-1 to 2-3 with theirrespective comparative examples:

FIG. 11 is a schematic view showing a surface hardening treatment deviceaccording to a third embodiment of the present invention:

FIG. 12 is a graph showing a control of furnace introduction gasesaccording to an example 3-2:

FIG. 13 is a graph showing a control of nitriding potential according tothe example 3-2;

FIG. 14 is a table comparing the examples 3-1 to 3-3 with theirrespective comparative examples:

FIG. 15 is a schematic view showing a surface hardening treatment deviceaccording to a fourth embodiment of the present invention:

FIG. 16 is a table comparing the examples 4-1 to 4-3 with theirrespective comparative examples;

FIG. 17 is a schematic view showing a surface hardening treatment deviceaccording to a fifth embodiment of the present invention:and

FIG. 18 is a table comparing the examples 5-1 to 5-3 with theirrespective comparative examples.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a preferable embodiment of the present invention will bedescribed. However, the present invention is not limited to theembodiment.

(Structure)

FIG. 1 is a schematic view showing a surface hardening treatment deviceaccording to an embodiment of the present invention. As shown in FIG. 1,the surface hardening treatment device 1 of the present embodiment is asurface hardening treatment device for performing a gas nitrocarburizingtreatment as a surface hardening treatment for a work S arranged in aprocessing furnace 2 by introducing an ammonia gas, an ammoniadecomposition gas and a carbon dioxide gas into the processing furnace2.

The ammonia decomposition gas is a gas called AX gas, and is a mixed gascomposed of nitrogen and hydrogen in a ratio of 1:3. The work S is madeof metal. For example, the work S is a steel part or a mold.

As shown in FIG. 1, the processing furnace 2 of the surface hardeningtreatment device 1 of the present embodiment includes: a stirring fan 8,a stirring-fan drive motor 9, a in-furnace temperature measuring device10, a furnace body heater 11, an atmospheric gas concentration detector3, a nitriding potential adjustor 4, a temperature adjustor 5, aprogrammable logic controller 31, a recorder 6, and a furnaceintroduction gas supplier 20.

The stirring fan 8 is disposed in the processing furnace 2 andconfigured to rotate in the processing furnace 2 in order to stiratmospheric gases in the processing furnace 2. The stirring-fan drivemotor 9 is connected to the stirring fan 8 and configured to cause thestirring fan 8 to rotate at an arbitrary rotation speed.

The in-furnace temperature measuring device 10 includes a thermocoupleand is configured to measure a temperature of the in-furnace gasesexisting in the processing furnace 2. In addition, after measuring thetemperature of the in-furnace gases, the in-furnace temperaturemeasuring device 10 is configured to output an information signalincluding the measured temperature (in-furnace temperature signal) tothe temperature adjustor 5 and the recorder 6.

The atmospheric gas concentration detector 3 is composed of: a sensorcapable of detecting a hydrogen concentration or an ammoniaconcentration in the processing furnace 2 as an in-furnace atmosphericgas concentration; and an oxygen sensor capable of detecting an oxygenconcentration in the processing furnace 2 as an in-furnace oxygenconcentration. A main body of each of the above sensors communicateswith an inside of the processing furnace 2 via an atmospheric gas pipe12. In the present embodiment, the atmospheric gas pipe 12 is formed asa single-line path that directly communicates the sensors' main bodiesof the atmospheric gas concentration detector 3 and the processingfurnace 2. An on-off valve 17 is provided in the middle of theatmospheric gas pipe 12. and configured to be controlled by an on-offvalve controller 16.

In addition, after detecting the in-furnace atmospheric gasconcentration and the in-furnace oxygen concentration, the atmosphericgas concentration detector 3 is configured to output an informationsignal including the detected concentrations to the nitriding potentialadjustor 4 and the recorder 6.

The recorder 6 includes a CPU and a storage medium such as a memory.Based on the signals outputted from the in-furnace temperaturemeasurement device 10 and the atmospheric gas concentration detector 3,the recorder 6 is configured to record the temperature and/or theatmospheric gas concentration and the oxygen concentration in theprocessing furnace 2, for example in correspondence with the date andtime when the surface hardening treatment is performed.

The nitriding potential adjuster 4 includes an in-furnace nitridingpotential calculator 13 and a gas flow rate output adjustor 30. Theprogrammable logic controller 31 includes a gas introduction controller14 and a parameter setting device 15.

The in-furnace nitriding potential calculator 13 is configured tocalculate a nitriding potential in the processing furnace 2 based on thehydrogen concentration or the ammonia concentration and the oxygenconcentration detected by the atmospheric gas concentration detector 3.Specifically, calculation formulas for the nitriding potential areprogrammed dependent on the actual furnace introduction gases inaccordance with the same theory as the above formulas (5) to (9), andincorporated in the in-furnace nitriding potential calculator 13, sothat the nitriding potential is calculated from the value of thein-furnace atmospheric gas concentration and the value of the oxygenconcentration.

In the present embodiment, the introduction amount C1 of the carbondioxide gas, which is a furnace introduction gas except for (other than)the ammonia gas and the ammonia decomposition gas, is controlled using afactor of proportionality c1 assigned to the carbon dioxide gas, suchthat C1=c1×(A+x=B), wherein the introduction amount of the ammonia gasis represented by A, the introduction amount of the ammoniadecomposition gas is represented by B, and a predetermined constant isrepresented by x.

The parameter setting device 15 is composed of a touch panel, forexample. Through the parameter setting device 15, the target nitridingpotential, the processing temperature, the processing time, theintroduction amount of the ammonia decomposition gas, the predeterminedconstant x, the factor of proportionality c1, and so on can be set andinputted for the same work. In addition, through the parameter settingdevice 15, setting parameter values for a PID control method can be setand inputted for each different value of the target nitriding potential.Specifically, “a proportional gain”, “an integral gain or an integrationtime”, and “a differential gain or a differentiation time” for the PIDcontrol method can be set and inputted for each different value of thetarget nitriding potential. The set and inputted setting parametervalues are transferred to the gas flow rate output adjustor 30.

The gas flow rate output adjustor 30 is configured to perform the PIDcontrol method in which respective gas introduction amounts of theammonia gas and the carbon dioxide gas among the three kinds of furnaceintroduction gases are input values, the nitriding potential calculatedby the in-furnace nitriding potential calculator 13 is an output value,and the target nitriding potential (the set nitriding potential) is atarget value. More specifically, in the present PID control method, thenitriding potential in the processing furnace 2 is brought close to thetarget nitriding potential by changing the introduction amount of theammonia gas and the introduction amount of the carbon dioxide gas whilekeeping the introduction amount of the ammonia decomposition gasconstant. In addition, in the present PID control method, the settingparameter values that have been transferred from the parameter settingdevice 15 are used.

Before the setting and inputting operation against the parameter settingdevice 15, it is preferable to perform pilot processes to obtain inadvance candidate values for the setting parameter values of the PIDcontrol method. According to the present embodiment, even if (1) a stateof the processing furnace (a state of a furnace wall and/or a jig), (2)a temperature condition of the processing furnace and (3) a state of thework (type and/or the number of parts) are the same, it is possible toobtain in advance candidate values for the setting parameter values (4)for each different value of the target nitriding potential, by anauto-tuning function that the nitriding potential adjustor 4 has initself. In order to embody the nitriding potential adjustor 4 havingsuch an auto-tuning function, a “UT75A” manufactured by YokogawaElectric Co., Ltd. (a high-functional digital indicating controller,http://www.yokogawa.co.jp/ns/cis/utup/utadvanced/ns-ut75a-01-ja.htm) orthe like can be used.

The setting parameter values (a set of “the proportional gain”, “theintegral gain or the integration time” and “the derivative gain or thederivative time”) obtained as the candidate values can be recorded insome manner, and then can be manually inputted to the parameter settingdevice 15. Alternatively, the setting parameter values obtained as thecandidate values can be stored in some storage device in a mannerassociated with the target nitriding potential, and then can beautomatically read out by the parameter setting device 15 based on theset and inputted value of the target nitriding potential.

Before performing the PID control method, the gas flow rate outputadjustor 30 is configured to determine an introduction amount of theammonia decomposition gas, which is kept constant, and respectiveinitial introduction amounts of the ammonia gas and of the carbondioxide gas, which are subsequently changed. It is preferable to performpilot processes to obtain in advance candidate values for theseintroduction amounts, so that the obtained values can be automaticallyread out by the parameter setting device 15 from some storage device orcan be manually inputted to the parameter setting device 15. Thereafter,according to the PID control method, the introduction amount of theammonia gas and the introduction amount of the carbon dioxide gas arechanged (while the introduction amount of the ammonia decomposition gasis kept constant) such that the nitriding potential in the processingfurnace 2 is brought close to the target nitriding potential while theabove relationship of C1=c1×(A+x×B) is maintained. The output valuesfrom the gas flow rate output adjustor 30 are transferred to the gasintroduction amount controller 14.

The gas introduction amount controller 14 is configured to transmit acontrol signal to a first supply amount controller 22 for the ammoniagas.

The furnace introduction gas supplier 20 of the present embodimentincludes a first furnace introduction gas supplier 21 for the ammoniagas, the first supply amount controller 22, a first supply valve 23 anda first flow meter 24. In addition, the furnace introduction gassupplier 20 of the present embodiment includes a second furnaceintroduction gas supplier 25 for the ammonia decomposition gas (AX gas),a second supply amount controller 26, a second supply valve 27 and asecond flow meter 28. Furthermore, the furnace introduction gas supplier20 of the present embodiment includes a third furnace introduction gassupplier 61 for the carbon dioxide gas, a third supply amount controller62, a third supply valve 63 and a third flow meter 64.

In the present embodiment, the ammonia gas, the ammonia decompositiongas and the carbon dioxide gas are mixed in a furnace introduction gaspipe 29 before entering the processing furnace 2.

The first furnace introduction gas supplier 21 is formed by, forexample, a tank filled with a first furnace introduction gas (in thisexample, the ammonia gas).

The first supply amount controller 22 is formed by a mass flowcontroller (which can finely change a flow rate within a short timeperiod), and is interposed between the first furnace introduction gassupplier 21 and the first supply valve 23. An opening degree of thefirst supply amount controller 22 changes according to the controlsignal outputted from the gas introduction amount controller 14, Inaddition, the first supply amount controller 22 is configured to detecta supply amount from the first furnace introduction gas supplier 21 tothe first supply valve 23, and output an information signal includingthe detected supply amount to the gas introduction amount controller 14and the recorder 6. This information signal can be used for correctionor the like of the control performed by the gas introduction amountcontroller 14.

The first supply valve 23 is formed by an electromagnetic valveconfigured to switch between opened and closed states according to acontrol signal outputted from the gas introduction amount controller 14,and is interposed between the first supply amount controller 22 and thefirst flow meter 24.

The first flow meter 24 is formed by, for example, a mechanical flowmeter such as a flow-type flow meter, and is interposed between thefirst supply valve 23 and the furnace introduction gas pipe 29. Thefirst flow meter 24 detects a supply amount from the first supply valve23 to the furnace introduction gas pipe 29. The supply amount detectedby the first flow meter 24 can be provided for an operators visualconfirmation.

The second furnace introduction gas supplier 25 is formed by, forexample, a tank filled with a second furnace introduction gas (in thisexample, the ammonia decomposition gas).

The second supply amount controller 26 is formed by a mass flowcontroller (which can finely change a flow rate within a short timeperiod), and is interposed between the second furnace introduction gassupplier 25 and the second supply valve 27. An opening degree of thesecond supply amount controller 26 changes according to the controlsignal outputted from the gas introduction amount controller 14. Inaddition, the second supply amount controller 26 is configured to detecta supply amount from the second furnace introduction gas supplier 25 tothe second supply valve 27, and output an information signal includingthe detected supply amount to the gas introduction amount controller 14and the recorder 6. This information signal can be used for correctionor the like of the control performed by the gas introduction amountcontroller 14.

The second supply valve 27 is formed by an electromagnetic valveconfigured to switch between opened and closed states according to acontrol signal outputted from the gas introduction amount controller 14,and is interposed between the second supply amount controller 26 and thesecond flow meter 28.

The second flow meter 28 is formed by, for example, a mechanical flowmeter such as a flow-type flow meter, and is interposed between thesecond supply valve 27 and the furnace introduction gas pipe 29. Thesecond flow meter 28 detects a supply amount from the second supplyvalve 27 to the furnace introduction gas pipe 29. The supply amountdetected by the second flow meter 28 can be provided for an operator'svisual confirmation.

Herein, in the present invention, the introduction amount of the ammoniadecomposition gas is not changed finely. Thus, the second supply amountcontroller 26 may be omitted, and a flow rate (an opening degree) of thesecond flow meter 28 may be manually adjusted correspondingly to thecontrol signal outputted from the gas introduction amount controller 14.

The third furnace introduction gas supplier 61 is formed by, forexample, a tank filled with a third furnace introduction gas (in thisexample, the carbon dioxide gas).

The third supply amount controller 62 is formed by a mass flowcontroller (which can finely change a flow rate within a short timeperiod), and is interposed between the third furnace introduction gassupplier 61 and the third supply valve 63. An opening degree of thethird supply amount controller 62 changes according to the controlsignal outputted from the gas introduction amount controller 14. Inaddition, the third supply amount controller 62 is configured to detecta supply amount from the third furnace introduction gas supplier 61 tothe third supply valve 63, and output an information signal includingthe detected supply amount to the gas introduction amount controller 14and the recorder 6. This information signal can be used for correctionor the like of the control performed by the gas introduction amountcontroller 14.

The third supply valve 63 is formed by an electromagnetic valveconfigured to switch between opened and closed states according to acontrol signal outputted from the gas introduction amount controller 14,and is interposed between the third supply amount controller 62 and thethird flow meter 64.

The third flow meter 64 is formed by, for example, a mechanical flowmeter such as a flow-type flow meter, and is interposed between thethird supply valve 63 and the furnace introduction gas pipe 29. Thethird flow meter 64 detects a supply amount from the third supply valve63 to the furnace introduction gas pipe 29. The supply amount detectedby the third flow meter 64 can be provided for an operator's visualconfirmation.

Operation: Example 1-1

Next, with reference to FIGS. 2 and 3, an operation of the surfacehardening treatment device 1 according to the present embodiment isexplained. First, a work S to be processed is put into the processingfurnace 2, and then the processing furnace 2 starts to be heated. In theexample shown in FIGS. 2 and 3, a pit furnace having a size of φ700×1000 was used as the processing furnace 2, 570° C. was adopted asthe temperature to be heated, and a steel material having a surface areaof 4 m² was used as the work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas and the carbon dioxide gas are introduced into theprocessing furnace 2 from the furnace introduction gas supplier 20according to their respective initial introduction amounts. In thisexample, as shown in FIG. 2, the initial introduction amount of theammonia gas was set to 13 [l/min], the initial introduction amount ofthe ammonia decomposition gas was set to 19 [l/min], the initialintroduction amount of the carbon dioxide gas was set to 1.03 [l/min],x=0.5 was set, and c1=0.053 was set. These initial introduction amountscan be set and inputted by the parameter setting device 15. Furthermore,the stirring fan drive motor 9 is driven and thus the stirring fan 8rotates to stir the atmospheric gases in the processing furnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17. In general, as a pretreatment for the gas nitriding treatment,a treatment for activating a steel surface to make it easy for nitrogento enter may be performed. In this case, a hydrogen chloride gas and/ora hydrogen cyanide gas or the like may be generated in the furnace.These gases may deteriorate the atmospheric gas concentration detector(sensors) 3, and thus it is effective to keep the on-off valve 17closed.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs art information insignal including the measured temperature to the nitriding potentialadjustor 4 and the recorder 6. The nitriding potential adjustor 4 judgeswhether the state in the processing furnace 2 is still during thetemperature rising step or already after the temperature rising step hasbeen completed (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (0.6 in this example: see FIG. 3) and astandard margin. This standard margin can also be set and inputted bythe parameter setting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (0.7 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and the soatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PID control method in which therespective gas introduction amounts of the ammonia gas and the carbondioxide gas among the three kinds of furnace introduction gases areinput values, the nitriding potential calculated by the in-furnacenitriding potential calculator 13 is an output value, and the targetnitriding potential (the set nitriding potential) is a target value.Specifically, in the present PID control method, the nitriding potentialin the processing furnace 2 is brought close to the target nitridingpotential while the relationship of C1=c1×(A+x×B) is maintained, bychanging the introduction amount of the ammonia gas and the introductionamount of the carbon dioxide gas while keeping the introduction amountof the ammonia decomposition gas constant. In the present PID controlmethod, the setting parameter values that have been set and inputted bythe parameter setting device 15 are used. The setting parameter valuesmay be different depending on values of the target nitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas and the introduction amount ofthe carbon dioxide gas as a result of the PID control method. The gasintroduction amount controller 14 transmits control signals to the firstsupply amount controller 22 for the ammonia gas, the second supplyamount controller 26 for the ammonia decomposition gas (whose flow rateis constant) and the third supply amount controller 62 for the carbondioxide gas, in order to realize the respective determined introductionamounts of the furnace introduction gases.

According to the control as described above, as shown in FIG. 3, thein-furnace nitriding potential can be stably controlled in the vicinityof the target nitriding potential. Thereby, the surface hardeningtreatment of the work S can be performed with extremely high quality. Asa specific example. in the example shown in FIGS. 2 and 3, a feedbackcontrol is performed with a sampling rate of about several hundredmilliseconds, and the introduction amount of the ammonia gas isincreased and decreased within a range of about 3 ml (±1.5 ml), so thatthe nitriding potential can be controlled to the target nitridingpotential (0.6) with extremely high precision since a timing of about 30minutes after starting the treatment. (In the example shown in FIGS. 2and 3, recording of the respective gas introduction amounts and thenitriding potential was stopped at a timing of about 190 minutes afterstarting the treatment.)

Operation: Example 1-2

Next, another case is explained as an example 1-2, in which the surfacehardening treatment device 1 according to the present embodiment is usedand the target nitriding potential is set to 0.4. In the example 1-2 aswell, a pit furnace having a size of φ 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas and the carbon dioxide gas are introduced into theprocessing furnace 2 from the furnace introduction gas supplier 20according to their respective initial introduction amounts. In thisexample, the initial introduction amount of the ammonia gas was set to5.5 [l/min], the initial introduction amount of the ammoniadecomposition gas was set to 25 [l/min], the initial introduction amountof the carbon dioxide gas was set to 0.95 [l/min], x=0.5 was set, andc1=0.053 was set. These initial introduction amounts can be set andinputted by the parameter setting device 15. Furthermore, the stirringfan drive motor 9 is driven and thus the stirring fan 8 rotates to stirthe atmospheric gases in the processing furnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (0.4 in this example) and a standard margin.This standard margin can also be set and inputted by the parametersetting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (0.5 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14, Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PID control method in which therespective gas introduction amounts of the ammonia gas and the carbondioxide gas among the three kinds of furnace introduction gases areinput values, the nitriding potential calculated in by the in-furnacenitriding potential calculator 13 is an output value, and the targetnitriding potential (the set nitriding potential) is a target value.Specifically, in the present PID control method, the nitriding potentialin the processing furnace 2 is brought close to the target nitridingpotential while the relationship of C1=c1×(A+x×B) is maintained, bychanging the introduction amount of the ammonia gas and the introductionamount of the carbon dioxide gas while keeping the introduction amountof the ammonia decomposition gas constant. In the present PID controlmethod, the setting parameter values that have been set and inputted bythe parameter setting device 15 are used. The setting parameter valuesmay be different depending on values of the target nitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas and the introduction amount ofthe carbon dioxide gas as a result of the PID control method. The gasintroduction amount controller 14 transmits control signals to the firstsupply amount controller 22 for the ammonia gas, the second supplyamount controller 26 for the ammonia decomposition gas (whose flow rateis constant) and the third supply amount controller 62 for the carbondioxide gas, in order to realize the respective determined introductionamounts of the furnace introduction gases.

According to the control as described above, the in-furnace nitridingpotential can be stably controlled in the vicinity of the targetnitriding potential. Thereby, the surface hardening treatment of thework S can be performed with extremely high quality. Specifically, afeedback control is performed with a sampling rate of about severalhundred milliseconds, and the introduction amount of the ammonia gas isincreased and decreased within a range of about 3 ml (±1.5 ml), so thatthe nitriding potential can be controlled to the target nitridingpotential (0.4) with extremely high precision since a timing of about 30minutes after starting the treatment.

Operation: Example 1-3

Next, further another case is explained as an example 1-3, in which thesurface hardening treatment device 1 according to the present embodimentis used and the target nitriding potential is set to 0.2. In the example1-3 as well, a pit furnace having a size of φ 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas and the carbon dioxide gas are introduced into theprocessing furnace 2 from the furnace introduction gas supplier 20according to their respective initial introduction amounts. In thisexample, as shown in FIG. 4, the initial introduction amount of theammonia gas was set to 2 [l/min], the initial introduction amount of theammonia decomposition gas was set to 29 [l/min], the initialintroduction amount of the carbon dioxide gas was set to 0.87 [l/min],x=0.5 was set, and c1=0.053 was set. These initial introduction amountscan be set and inputted by the parameter setting device 15. Furthermore,the stirring fan drive motor 9 is driven and thus the stirring fan 8rotates to stir the atmospheric gases in the processing furnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (0.2 in this example: see FIG. 5) and astandard margin. This standard margin can also be set and inputted bythe parameter setting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (0.3 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PID control method in which therespective gas introduction amounts of the ammonia gas and the carbondioxide gas among the three kinds of furnace introduction gases areinput values, the nitriding potential calculated by the in-furnacenitriding potential calculator 13 is an output value, and the targetnitriding potential (the set nitriding potential) is a target value.Specifically, in the present PID control method, the nitriding potentialin the processing furnace 2 is brought close to the target nitridingpotential while the relationship of C1=c1×(A+x×B) is maintained, bychanging the introduction amount of the ammonia gas and the introductionamount of the carbon dioxide gas while keeping the introduction amountof the ammonia decomposition gas constant. In the present PID controlmethod, the setting parameter values that have been set and inputted bythe parameter setting device 15 are used. The setting parameter valuesmay be different depending on values of the target nitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas and the introduction amount ofthe carbon dioxide gas as a result of the PID control method. The gasintroduction amount controller 14 transmits control signals to the firstsupply amount controller 22 for the ammonia gas, the second supplyamount controller 26 for the ammonia decomposition gas (whose flow rateis constant) and the third supply amount controller 62 for the carbondioxide gas, in order to realize the respective determined introductionamounts of the furnace introduction gases.

According to the control as described above, as shown in FIG. 5, thein-furnace nitriding potential can be stably controlled in the vicinityof the target nitriding potential. Thereby, the surface hardeningtreatment of the work S can be performed with extremely high quality. Asa specific example, in the example shown in FIGS. 4 and 5, a feedbackcontrol is performed with a sampling rate of about several hundredmilliseconds, and the introduction amount of the ammonia gas isincreased and decreased within a range of about 3 ml (±1.5 ml), so thatthe nitriding potential can be controlled to the target nitridingpotential (0.2) with extremely high precision since a timing of about 30minutes after starting the treatment. (In the example shown in FIGS. 4and 5, recording of the respective gas introduction amounts and thenitriding potential was stopped at a timing of about 160 minutes afterstarting the treatment.)

(Explanation of Comparative Examples)

As comparative examples, controls of nitriding potential were performed.In each of them, the ammonia decomposition gas was not introduced, theratio of the introduction amounts of the ammonia gas and the carbondioxide gas was always maintained at 95:5, and the total introductionamount thereof was changed.

Specifically, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculated the in-furnace nitridingpotential based on the inputted hydrogen concentration signal or ammoniaconcentration signal and the inputted oxygen concentration signal. Then,the gas flow rate output adjustor 30 performed the PID control method inwhich the respective gas introduction amounts of the ammonia gas and thecarbon dioxide gas were input values, the nitriding potential calculatedby the in-furnace nitriding potential calculator 13 was an output value,and the target nitriding potential (the set nitriding potential) was atarget value. More specifically, in the present PID control method, thenitriding potential in the processing furnace 2 was brought close to thetarget nitriding potential, by changing the total introduction amount ofthe ammonia gas and the carbon dioxide gas while keeping the ratio ofthe introduction amounts of the ammonia gas and the carbon dioxide gasconstant.

However, in the above comparative examples, the nitriding potentialcould not be stably controlled.

Comparison Between Examples 1-1 to 1-3 and Comparative Examples

A table of the above results is shown as FIG. 6.

(Structure of Second Embodiment)

As shown in FIG. 7, in a second embodiment, a third furnace introductiongas supplier 61′ is formed by a tank filled with not a carbon dioxidegas but a carbon monoxide gas.

In the second embodiment, the introduction amount C1 of the carbonmonoxide gas, which is a furnace introduction gas except for (otherthan) the ammonia gas and the ammonia decomposition gas, is controlledusing a factor of proportionality c1 assigned to the carbon monoxidegas, such that C1=c1×(A+x×B), wherein the introduction amount of theammonia gas is represented by A, the introduction amount of the ammoniadecomposition gas is represented by B, and a predetermined constant isrepresented by x.

The other structure of the second embodiment is substantially the sameas that of the first embodiment explained with reference to FIG. 1. InFIG. 7, the same parts as those of the first embodiment are shown by thesame reference numerals, and detailed explanation thereof is omitted.

Operation: Example 2-1

Next, a case is explained as an example 2-1, in which the surfacehardening treatment device according to the second embodiment is usedand the target nitriding potential is set to 0.6. In the example 2-1 aswell, a pit furnace having a size of φ 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas and the carbon monoxide gas are introduced into theprocessing furnace 2 from the furnace introduction gas supplier 20according to their respective initial introduction amounts. In thisexample, the initial introduction amount of the ammonia gas was set to5.5 [l/min], the initial introduction amount of the ammoniadecomposition gas was set to 19 [l/min], the initial introduction amountof the carbon monoxide gas was set to 0.2 [l/min], x=0.5 was set, andc1=0.01 was set. These initial introduction amounts can be set andinputted by the parameter setting device 15. Furthermore, the stirringfan drive motor 9 is driven and thus the stirring fan 8 rotates to stirthe atmospheric gases in the processing furnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (0.6 in this example) and a standard margin.This standard margin can also be set and inputted by the parametersetting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (0.7 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PID control method in which therespective gas introduction amounts of the ammonia gas and the carbonmonoxide gas among the three kinds of furnace introduction gases areinput values, the nitriding potential calculated by the in-furnacenitriding potential calculator 13 is an output value, and the targetnitriding potential (the set nitriding potential) is a target value.Specifically, in the present PID control method, the nitriding potentialin the processing furnace 2 is brought close to the target nitridingpotential while the relationship of C1=c1×(A+x×B) is maintained, bychanging the introduction amount of the ammonia gas and the introductionamount of the carbon monoxide gas while keeping the introduction amountof the ammonia decomposition gas constant. In the present PID controlmethod, the setting parameter values that have been set and inputted bythe parameter setting device 15 are used. The setting parameter valuesmay be different depending on values of the target nitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas and the introduction amount ofthe carbon monoxide gas as a result of the PID control method. The gasintroduction amount controller 14 transmits control signals to the firstsupply amount controller 22 for the ammonia gas, the second supplyamount controller 26 for the ammonia decomposition gas (whose flow rateis constant) and the third supply amount controller 62 for the carbonmonoxide gas, in order to realize the respective determined introductionamounts of the furnace introduction gases.

According to the control as described above, the in-furnace nitridingpotential can be stably controlled in the vicinity of the targetnitriding potential. Thereby, the surface hardening treatment of thework S can be performed with extremely high quality. Specifically, afeedback control is performed with a sampling rate of about severalhundred milliseconds, and the introduction amount of the ammonia gas isincreased and decreased within a range of about 3 ml (±1.5 ml), so thatthe nitriding potential can be controlled to the target nitridingpotential (0.6) with extremely high precision since a timing of about 20minutes after starting the treatment.

Operation: Example 2-2

Next, another case is explained as an example 2-2, in which the surfacehardening treatment device according to the second embodiment is usedand the target nitriding potential is set to 0.4. In the example 2-2 aswell, a pit furnace having a size of φ 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas and the carbon monoxide gas are introduced into theprocessing furnace 2 from the furnace introduction gas supplier 20according to their respective initial introduction amounts. In thisexample, as shown in FIG. 8, the initial introduction amount of theammonia gas was set to 3 [l/min], the initial introduction amount of theammonia decomposition gas was set to 25 [l/min], the initialintroduction amount of the carbon monoxide gas was set to 0.15 [l/min].x=0.5 was set, and c1=0.01 was set. These initial introduction amountscan be set and inputted by the parameter setting device 15. Furthermore,the stirring fan drive motor 9 is driven and thus the stirring fan 8rotates to stir the atmospheric gases in the processing furnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4so and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (0.4 in this example) and a standard margin.This standard margin can also be set and inputted by the parametersetting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (0.5 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PID control method in which therespective gas introduction amounts of the ammonia gas and the carbonmonoxide gas among the three kinds of furnace introduction gases areinput values, the nitriding potential calculated by the in-furnacenitriding potential calculator 13 is an output value, and the targetnitriding potential (the set nitriding potential) is a target value.Specifically, in the present PID control method, the nitriding potentialin the processing furnace 2 is brought close to the target nitridingpotential while the relationship of C1=c1×(A+x×B) is maintained, bychanging the introduction amount of the ammonia gas and the introductionamount of the carbon monoxide gas while keeping the introduction amountof the ammonia decomposition gas constant. In the present PID controlmethod, the setting parameter values that have been set and inputted bythe parameter setting device 15 are used. The setting parameter valuesmay be different depending on values of the target nitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas and the introduction amount ofthe carbon monoxide gas as a result of the PID control method. The gasintroduction amount controller 14 transmits control signals to the firstsupply amount controller 22 for the ammonia gas. the second supplyamount controller 26 for the ammonia decomposition gas (whose flow rateis constant) and the third supply amount controller 62 for the carbonmonoxide gas, in order to realize the respective determined introductionamounts of the furnace introduction gases.

According to the control as described above, as shown in FIG. 9, thein-furnace nitriding potential can be stably controlled in the vicinityof the target nitriding potential. Thereby, the surface hardeningtreatment of the work S can be performed with extremely high quality.Specifically, a feedback control is performed with a sampling rate ofabout several hundred milliseconds, and the introduction amount of theammonia gas is increased and decreased within a range of about 3 ml(±1.5 ml), so that the nitriding potential can be controlled to thetarget nitriding potential (0.4) with extremely high precision since atiming of about 20 minutes after starting the treatment.

Operation: Example 2-3

Next, another case is explained as an example 2-3, in which the surfacehardening treatment device according to the second embodiment is usedand the target nitriding potential is set to 0.2. In the example 2-3 aswell, a pit furnace having a size of φ 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas and the carbon monoxide gas are introduced into theprocessing furnace 2 from the furnace introduction gas supplier 20according to their respective initial introduction amounts. In thisexample, the initial introduction amount of the ammonia gas was set to 1[l/min], the initial introduction amount of the ammonia decompositiongas was set to 29 [l/min], the initial introduction amount of the carbonmonoxide gas was set to 0.15 [l/min]. x=0.5 was set, and c1=0.01 wasset. These initial introduction amounts can be set and inputted by theparameter setting device 15. Furthermore, the stirring fan drive motor 9is driven and thus the stirring fan 8 rotates to stir the atmosphericgases in the processing furnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (0.3 in this example) and a standard margin.This standard margin can also be set and inputted by the parametersetting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (0.4 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PID control method in which therespective gas introduction amounts of the ammonia gas and the carbonmonoxide gas among the three kinds of furnace introduction gases areinput values, the nitriding potential calculated by the in-furnacenitriding potential calculator 13 is an output value, and the targetnitriding potential (the set nitriding potential) is a target value.Specifically, in the present PID control method, the nitriding potentialin the processing furnace 2 is brought close to the target nitridingpotential while the relationship of C1=c1×(A+x×B) is maintained, bychanging the introduction amount of the ammonia gas and the introductionamount of the carbon monoxide gas while keeping the introduction amountof the ammonia decomposition gas constant. In the present PID controlmethod, the setting parameter values that have been set and inputted bythe parameter setting device 15 are used. The setting parameter valuesmay be different depending on values of the target nitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas and the introduction amount ofthe carbon monoxide gas as a result of the PID control method. The gasin introduction amount controller 14 transmits control signals to thefirst supply amount controller 22 for the ammonia gas, the second supplyamount controller 26 for the ammonia decomposition gas (whose flow rateis constant) and the third supply amount controller 62 for the carbonmonoxide gas, in order to realize the respective determined introductionamounts of the furnace introduction gases.

According to the control as described above, the in-furnace nitridingpotential can be stably controlled in the vicinity of the targetnitriding potential. Thereby, the surface hardening treatment of thework S can be performed with extremely high quality. Specifically, afeedback control is performed with a sampling rate of about severalhundred milliseconds, and the introduction amount of the ammonia gas isincreased and decreased within a range of about 3 ml (±1.5 ml), so thatthe nitriding potential can be controlled to the target nitridingpotential (0.2) with extremely high precision since a timing of about 30minutes after starting the treatment.

Explanation of Comparative Examples

As comparative examples, controls of nitriding potential were performed.In each of them, the ammonia decomposition gas was not introduced, theratio of the introduction amounts of the ammonia gas and the carbonmonoxide gas was always maintained at 99:1, and the total introductionamount thereof was changed.

Specifically, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculated the in-furnace nitridingpotential based on the inputted hydrogen concentration signal or ammoniaconcentration signal and the inputted oxygen concentration signal. Then,the gas flow rate output adjustor 30 performed the PID control method inwhich the respective gas introduction amounts of the ammonia gas and thecarbon monoxide gas were input values, the nitriding potentialcalculated by the in-furnace nitriding potential calculator 13 was anoutput value, and the target nitriding potential (the set nitridingpotential) was a target value. More specifically, in the present MDcontrol method, the nitriding potential in the processing furnace 2 wasbrought close to the target nitriding potential, by changing the totalintroduction amount of the ammonia gas and the carbon monoxide gas whilekeeping the ratio of the introduction amounts of the ammonia gas and thecarbon monoxide gas constant.

However, in the above comparative examples, the nitriding potentialcould not be stably controlled.

Comparison Between Examples 2-1 to 2-3 and Comparative Examples

A table of the above results is shown as FIG. 10.

(Structure of Third Embodiment)

As shown in FIG. 11, a furnace introduction gas supplier 20′ of a thirdembodiment further includes a fourth furnace introduction gas supplier71 for nitrogen gas, a fourth supply amount controller 72, a fourthsupply valve 73 and a fourth flow meter 74.

The fourth furnace introduction gas supplier 71 is formed by, forexample, a tank filled with a fourth furnace introduction gas (in thisexample, nitrogen gas).

The fourth supply amount controller 72 is formed by a mass flowcontroller (which can finely change a flow rate within a short timeperiod), and is interposed between the fourth furnace introduction gassupplier 71 and the fourth supply valve 73. An opening degree of thefourth supply amount controller 72 changes according to the controlsignal outputted from the gas introduction amount controller 14. Inaddition, the fourth supply amount controller 72 is configured to detecta supply amount from the fourth furnace introduction gas supplier 71 tothe fourth supply valve 73, and output an information signal includingthe detected supply amount to the gas introduction amount controller 14and the recorder 6. This information signal can be used for correctionor the like of the control performed by the gas introduction amountcontroller 14.

The fourth supply valve 73 is formed by an electromagnetic valveconfigured to switch between opened and closed states according to acontrol signal outputted from the gas introduction amount controller 14,and is interposed between the fourth supply amount controller 72 and thefourth flow meter 74.

The fourth flow meter 74 is formed by, for example, a mechanical flowmeter such as a flow-type flow meter, and is interposed between thefourth supply valve 73 and the furnace introduction gas pipe 29. Thefourth flow meter 74 detects a supply amount from the fourth supplyvalve 73 to the furnace introduction gas pipe 29. The supply amountdetected by the fourth flow meter 74 can be provided for an operator'svisual confirmation.

In the third embodiment, the introduction amount C1 of the carbondioxide gas and the introduction amount C2 of the nitrogen gas, whichare furnace introduction gases except for (other than) the ammonia gasand the ammonia decomposition gas, are controlled using a factor ofproportionality c1 assigned to the carbon dioxide gas and a factor ofproportionality c2 assigned to the nitrogen gas, such that C1=c1×(A+x×B)and C2=c2×(A+x×B), wherein the introduction amount of the ammonia gas isrepresented by A, the introduction amount of the ammonia decompositiongas is represented by B, and a predetermined constant is represented byx.

The other structure of the third embodiment is substantially the same asthat of the first embodiment explained with reference to FIG. 1. In FIG.11, the same parts as those of the first embodiment are shown by thesame reference numerals, and detailed explanation thereof is omitted.)

Operation: Example 3-1

Next, a case is explained as an example 3-1, in which the surfacehardening treatment device according to the third embodiment is used andthe target nitriding potential is set to 1.0. In the example 3-1 aswell, a pit furnace having a size of φ 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas, the carbon dioxide gas and the nitrogen gas areintroduced into the processing furnace 2 from the furnace introductiongas supplier 20′ according to their respective initial introductionamounts. In this example, the initial introduction amount of the ammoniagas was set to 13 [l/min], the initial introduction amount of theammonia decomposition gas was set to 19 [l/min], the initialintroduction amount of the carbon dioxide gas was set to 2.2 [l/min],the initial introduction amount of the nitrogen gas was set to 20[l/min], x=0.5 was set, c1=0.1 was set, and c2=0.9 was set. Theseinitial introduction amounts can be set and inputted by the parametersetting device 15. Furthermore, the stirring fan drive motor 9 is drivenand thus the stirring fan 8 rotates to stir the atmospheric gases in theprocessing furnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (1.0 in this example) and a standard margin.This standard margin can also be set and inputted by the parametersetting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (1.1 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the Pill control method in which therespective gas introduction amounts of the ammonia gas, the carbondioxide gas and the nitrogen gas among the four kinds of furnaceintroduction gases are input values, the nitriding potential calculatedby the in-furnace nitriding potential calculator 13 is an output value,and the target nitriding potential (the set nitriding potential) is atarget value. Specifically, in the present PID control method, thenitriding potential in the processing furnace 2 is brought close to thetarget nitriding potential while the relationships of C1=c1×(A+x×B) andC2=c2×(A+x×B) are maintained, by changing the introduction amount of theammonia gas, the introduction amount of the carbon dioxide gas and theintroduction amount of the nitrogen gas while keeping the introductionamount of the ammonia decomposition gas constant. In the present PIDcontrol method, the setting parameter values that have been set andinputted by the parameter setting device 15 are used. The settingparameter values may be different depending on values of the targetnitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas, the introduction amount of thecarbon dioxide gas and the introduction amount of the nitrogen gas as aresult of the PID control method. The gas introduction amount controller14 transmits control signals to the first supply amount controller 22for the ammonia gas, the second supply amount controller 26 for theammonia decomposition gas (whose flow rate is constant), the thirdsupply amount controller 62 for the carbon dioxide gas and the fourthsupply amount controller 72 for the nitrogen gas, in order to realizethe respective determined introduction amounts of the furnaceintroduction gases.

According to the control as described above, the in-furnace nitridingpotential can be stably controlled in the vicinity of the targetnitriding potential. Thereby, the surface hardening treatment of thework S can be performed with extremely high quality. Specifically, afeedback control is performed with a sampling rate of about severalhundred milliseconds, and the introduction amount of the ammonia gas isincreased and decreased within a range of about 3 ml (±1.5 ml), so thatthe nitriding potential can be controlled to the target nitridingpotential (1.0) with extremely high precision since a timing of about 20minutes after starting the treatment.

Operation: Example 3-2

Next, another case is explained as an example 3-2, in which the surfacehardening treatment device according to the third embodiment is used andthe target nitriding potential is set to 0.6. In the example 3-2 aswell, a pit furnace having a size of φ 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas, the carbon dioxide gas and the nitrogen gas areintroduced into the processing furnace 2 from the furnace introductiongas supplier 20′ according to their respective initial introductionamounts. In this example, as shown in FIG. 12, the initial introductionamount of the ammonia gas was set to 8 [l/min], the initial introductionamount of the ammonia decomposition gas was set to 25 [l/min], theinitial introduction amount of the carbon dioxide gas was set to 2[l/min], the initial introduction amount of the nitrogen gas was set to18.5 [l/min], x=0.5 was set, c1=0.1 was set, and c2=0.9 was set. Theseinitial introduction amounts can be set and inputted by the parametersetting device 15. Furthermore, the stirring fan drive motor 9 is drivenand thus the stirring fan 8 rotates to stir the atmospheric gases in theprocessing furnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (0.6 in this example) and a standard margin.This standard margin can also be set and inputted by the parametersetting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (0.7 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PO control method in which therespective gas introduction amounts of the ammonia gas, the carbondioxide gas and the nitrogen gas among the four kinds of furnaceintroduction gases are input values, the nitriding potential calculatedby the in-furnace nitriding potential calculator 13 is an output value,and the target nitriding potential (the set nitriding potential) is atarget value. Specifically, in the present PID control method, thenitriding potential in the processing furnace 2 is brought close to thetarget nitriding potential while the relationships of C1=c1×(A+x×B) andC2=c2×(A+x×B) are maintained, by changing the introduction amount of theammonia gas, the introduction amount of the carbon dioxide gas and theintroduction amount of the nitrogen gas while keeping the introductionamount of the ammonia decomposition gas constant. In the present PIDcontrol method, the setting parameter values that have been set andinputted by the parameter setting device 15 are used. The settingparameter values may be different depending on values of the targetnitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas, the introduction amount of thecarbon dioxide gas and the introduction amount of the nitrogen gas as aresult of the PID control method. The gas introduction amount controller14 transmits control signals to the first supply amount controller 22for the ammonia gas, the second supply amount controller 26 for theammonia decomposition gas (whose flow rate is constant), the thirdsupply amount controller 62 for the carbon dioxide gas and the fourthsupply amount controller 72 for the nitrogen gas, in order to realizethe respective determined introduction amounts of the furnaceintroduction gases.

According to the control as described above, as shown in FIG. 13, thein-furnace nitriding potential can be stably controlled in the vicinityof the target nitriding potential. Thereby, the surface hardeningtreatment of the work S can be performed with extremely high quality.Specifically, a feedback control is performed with a sampling rate ofabout several hundred milliseconds, and the introduction amount of theammonia gas is increased and decreased within a range of about 3 ml(±1.5 ml), so that the nitriding potential can be controlled to thetarget nitriding potential (0.6) with extremely high precision since atiming of about 30 minutes after starting the treatment.

Operation: Example 3-3

Next, another case is explained as an example 3-3, in which the surfacehardening treatment device according to the third embodiment is used andthe target nitriding potential is set to 0.2. In the example 3-3 aswell, a pit furnace having a size of 9 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas, the carbon dioxide gas and the nitrogen gas areintroduced into the processing furnace 2 from the furnace introductiongas supplier 20′ according to their respective initial introductionamounts. In this example, the initial introduction amount of the ammoniagas was set to 3 [l/min], the initial introduction amount of the ammoniadecomposition gas was set to 29 [l/min], the initial introduction amountof the carbon dioxide gas was set to 1.8 [l/min], the initialintroduction amount of the nitrogen gas was set to 15.8 [l/min], x=0.5was set, c1=0.1 was set, and c2=0.9 was set. These initial introductionamounts can be set and inputted by the parameter setting device 15.Furthermore, the stirring fan drive motor 9 is driven and thus thestirring fan 8 rotates to stir the atmospheric gases in the processingfurnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (0.2 in this example) and a standard margin.This standard margin can also be set and inputted by the parametersetting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (0.3 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PID control method in which therespective gas introduction amounts of the ammonia gas, the carbondioxide gas and the nitrogen gas among the four kinds of furnaceintroduction gases are input values, the nitriding potential calculatedby the in-furnace nitriding potential calculator 13 is an output value,and the target nitriding potential (the set nitriding potential) is atarget value. Specifically, in the present PID control method, thenitriding potential in the processing furnace 2 is brought close to thetarget nitriding potential while the relationships of C1=c1×(A+x×B) andC2=c2×(A+x×B) are maintained, by changing the introduction amount of theammonia gas, the introduction amount of the carbon dioxide gas and theintroduction amount of the nitrogen gas while keeping the introductionamount of the ammonia decomposition gas constant. In the present PIDcontrol method, the setting parameter values that have been set andinputted by the parameter setting device 15 are used. The settingparameter values may be different depending on values of the targetnitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas, the introduction amount of thecarbon dioxide gas and the introduction amount of the nitrogen gas as aresult of the PID control method. The gas introduction amount controller14 transmits control signals to the first supply amount controller 22for the ammonia gas, the second supply amount controller 26 for theammonia decomposition gas (whose flow rate is constant), the thirdsupply amount controller 62 for the carbon dioxide gas and the fourthsupply amount controller 72 for the nitrogen gas, in order to realizethe respective determined introduction amounts of the furnaceintroduction gases.

According to the control as described above, the in-furnace nitridingpotential can be stably controlled in the vicinity of the targetnitriding potential. Thereby, the surface hardening treatment of thework S can be performed with extremely high quality. Specifically, afeedback control is performed with a sampling rate of about severalhundred milliseconds, and the introduction amount of the ammonia gas isincreased and decreased within a range of about 3 ml (±1.5 ml), so thatthe nitriding potential can be controlled to the target nitridingpotential (0.2) with extremely high precision since a timing of about 40minutes after starting the treatment.)

Explanation of Comparative Examples

As comparative examples, controls of nitriding potential were performed.In each of them, the ammonia decomposition gas was not introduced, theratio of the introduction amounts of the ammonia gas, the nitrogen gasand the carbon dioxide gas was always maintained at 50:45:5, and thetotal introduction amount thereof was changed.

Specifically, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculated the in-furnace nitridingpotential based on the inputted hydrogen concentration signal or ammoniaconcentration signal and the inputted oxygen concentration signal. Then,the gas flow rate output adjustor 30 performed the PID control method inwhich the respective gas introduction amounts of the ammonia gas, thenitrogen gas and the carbon dioxide gas were input values, the nitridingpotential calculated by the in-furnace nitriding potential calculator 13was an output value, and the target nitriding potential (the setnitriding potential) was a target value. More specifically, in thepresent PID control method, the nitriding potential in the processingfurnace 2 was brought close to the target nitriding potential, bychanging the total introduction amount of the ammonia gas, the nitrogengas and the carbon dioxide gas while keeping the ratio of theintroduction amounts of the ammonia gas, the nitrogen gas and the carbondioxide gas constant.

However, in the above comparative examples, the nitriding potentialcould not be stably controlled.

Comparison Between Examples 3-1 to 3-3 and Comparative Examples

A table of the above results is shown as FIG. 14.

(Structure of Fourth Embodiment)

As shown in FIG. 15, in a fourth embodiment, a third furnaceintroduction gas supplier 61′ is formed by a tank filled with not acarbon dioxide gas but a carbon monoxide gas.

In the fourth embodiment, the introduction amount C1 of the carbonmonoxide gas and the introduction amount C2 of the nitrogen gas, whichare furnace introduction gases except for (other than) the ammonia gasand the ammonia decomposition gas, are controlled using a factor ofproportionality c1 assigned to the carbon monoxide gas and a factor ofproportionality c2 assigned to the nitrogen gas, such that C1=c1×(A+x×B)and C2=c2×(A+x×B), wherein the introduction amount of the ammonia gas isrepresented by A, the introduction amount of the ammonia decompositiongas is represented by B, and a predetermined constant is represented byx.

The other structure of the fourth embodiment is substantially the sameas that of the third embodiment explained with reference to FIG. 11. InFIG. 15, the same parts as those of the third embodiment are shown bythe same reference numerals, and detailed explanation thereof isomitted.

Operation: Example 4-1

Next, a case is explained as an example 4-1, in which the surfacehardening treatment device according to the fourth embodiment is usedand the target nitriding potential is set to 1.0. In the example 4-1 aswell, a pit furnace having a size of φ 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas, the carbon monoxide gas and the nitrogen gas areintroduced into the processing furnace 2 from the furnace introductiongas supplier 20′ according to their respective initial introductionamounts. In this example, the initial introduction amount of the ammoniagas was set to 13 [l/min], the initial introduction amount of theammonia decomposition gas was set to 19 [l/min], the initialintroduction amount of the carbon monoxide gas was set to 0.9 [l/min],the initial introduction amount of the nitrogen gas was set to 20[l/min], x=0.5 was set, c1=0.04 was set, and c2=0.96 was set. Theseinitial introduction amounts can be set and inputted by the parametersetting device 15. Furthermore, the stirring fan drive motor 9 is drivenand thus the stirring fan 8 rotates to stir the atmospheric gases in theprocessing furnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (1.0 in this example) and a standard margin.This standard margin can also be set and inputted by the parametersetting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (1.1 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PID control method in which therespective gas introduction amounts of the ammonia gas, the carbonmonoxide gas and the nitrogen gas among the four kinds of furnaceintroduction gases are input values, the nitriding potential calculatedby the in-furnace nitriding potential calculator 13 is an output value,and the target nitriding potential (the set nitriding potential) is atarget value. Specifically, in the present PID control method, thenitriding potential in the processing furnace 2 is brought close to thetarget nitriding potential while the relationships of C1=c1×(A+x×B) andC2=c2×(A+x×B) are maintained, by changing the introduction amount of theammonia gas, the introduction amount of the carbon monoxide gas and theintroduction amount of the nitrogen gas while keeping the introductionamount of the ammonia decomposition gas constant. In the present PIDcontrol method, the setting parameter values that have been set andinputted by the parameter setting device 15 are used. The settingparameter values may be different depending on values of the targetnitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas, the introduction amount of thecarbon monoxide gas and the introduction amount of the nitrogen gas as aresult of the PID control method. The gas introduction amount controller14 transmits control signals to the first supply amount controller 22for the ammonia gas, the second supply amount controller 26 for theammonia decomposition gas (whose flow rate is constant), the thirdsupply amount controller 62 for the carbon monoxide gas and the fourthsupply amount controller 72 for the nitrogen gas, in order to realizethe respective determined introduction amounts of the furnaceintroduction gases.

According to the control as described above, the in-furnace nitridingpotential can be stably controlled in the vicinity of the targetnitriding potential. Thereby, the surface hardening treatment of thework S can be performed with extremely high quality. Specifically, afeedback control is performed with a sampling rate of about severalhundred milliseconds, and the introduction amount of the ammonia gas isincreased and decreased within a range of about 3 ml (±1.5 ml), so thatthe nitriding potential can be controlled to the target nitridingpotential (1.0) with extremely high precision since a timing of about 30minutes after starting the treatment.

Operation: Example 4-2

Next, another case is explained as an example 4-2, in which the surfacehardening treatment device according to the fourth embodiment is usedand the target nitriding potential is set to 0.6. In the example 4-2 aswell, a pit furnace having a size of φ 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas, the carbon monoxide gas and the nitrogen gas areintroduced into the processing furnace 2 from the furnace introductiongas supplier 20′ according to their respective initial introductionamounts. In this example, the initial introduction amount of the ammoniagas was set to 8 [l/min], the initial introduction amount of the ammoniadecomposition gas was set to 25 [l/min], the initial introduction amountof the carbon monoxide gas was set to 0.8 [l/min], the initialintroduction amount of the nitrogen gas was set to 19.7 [l/min], x=0.5was set, c1=0.04 was set, and c2=0.96 was set. These initialintroduction amounts can be set and inputted by the parameter settingdevice 15. Furthermore, the stirring fan drive motor 9 is driven andthus the stirring fan 8 rotates to stir the atmospheric gases in theprocessing furnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (0.6 in this example) and a standard margin.This standard margin can also be set and inputted by the parametersetting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (0.7 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PID control method in which therespective gas introduction amounts of the ammonia gas, the carbonmonoxide gas and the nitrogen gas among the four kinds of furnaceintroduction gases are input values, the nitriding potential calculatedby the in-furnace nitriding potential calculator 13 is an output value,and the target nitriding potential (the set nitriding potential) is atarget value. Specifically, in the present PID control method, thenitriding potential in the processing furnace 2 is brought close to thetarget nitriding potential while the relationships of C1=c1×(A+x×B) andC2=c2×(A+x×B) are maintained, by changing the introduction amount of theammonia gas, the introduction amount of the carbon monoxide gas and theintroduction amount of the nitrogen gas while keeping the introductionamount of the ammonia decomposition gas constant. In the present PIDcontrol method, the setting parameter values that have been set andinputted by the parameter setting device 15 are used. The settingparameter values may be different depending on values of the targetnitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas, the introduction amount of thecarbon monoxide gas and the introduction amount of the nitrogen gas as aresult of the PID control method. The gas introduction amount controller14 transmits control signals to the first supply amount controller 22for the ammonia gas, the second supply amount controller 26 for theammonia decomposition gas (whose flow rate is constant), the thirdsupply amount controller 62 for the carbon monoxide gas and the fourthsupply amount controller 72 for the nitrogen gas, in order to realizethe respective determined introduction amounts of the furnaceintroduction gases.

According to the control as described above, the in-furnace nitridingpotential can be stably controlled in the vicinity of the targetnitriding potential. Thereby, the surface hardening treatment of thework S can be performed with extremely high quality. Specifically, afeedback control is performed with a sampling rate of about severalhundred milliseconds, and the introduction amount of the ammonia gas isincreased and decreased within a range of about 3 ml (±1.5 ml), so thatthe nitriding potential can be controlled to the target nitridingpotential (0.6) with extremely high precision since a timing of about 40minutes after starting the treatment.

Operation: Example 4-3

Next, another case is explained as an example 4-3, in which the surfacehardening treatment device according to the fourth embodiment is usedand the target nitriding potential is set to 0.2. In the example 4-3 aswell, a pit furnace having a size of φ 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas, the carbon monoxide gas and the nitrogen gas areintroduced into the processing furnace 2 from the furnace introductiongas supplier 20′ according to their respective initial introductionamounts. In this example, the initial introduction amount of the ammoniagas was set to 3 [l/min], the initial introduction amount of the ammoniadecomposition gas was set to 29 [l/min], the initial introduction amountof the carbon monoxide gas was set to 0.7 [l/min], the initialintroduction amount of the nitrogen gas was set to 16 [l/min], x=0.5 wasset, c1=0.04 was set, and c2=0.96 was set. These initial introductionamounts can be set and inputted by the parameter setting device 15.Furthermore, the stirring fan drive motor 9 is driven and thus thestirring fan 8 rotates to stir the atmospheric gases in the processingfurnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.)

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (0.2 in this example) and a standard margin.This standard margin can also be set and inputted by the parametersetting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (0.3 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PID control method in which therespective gas introduction amounts of the ammonia gas, the carbonmonoxide gas and the nitrogen gas among the four kinds of furnaceintroduction gases are input values, the nitriding potential calculatedby the in-furnace nitriding potential calculator 13 is an output value,and the target nitriding potential (the set nitriding potential) is atarget value. Specifically, in the present PID control method, thenitriding potential in the processing furnace 2 is brought close to thetarget nitriding potential while the relationships of C1=c1×(A+x×B) andC2=c2×(A+x×B) are maintained, by changing the introduction amount of theammonia gas, the introduction amount of the carbon monoxide gas and theintroduction amount of the nitrogen gas while keeping the introductionamount of the ammonia decomposition gas constant. In the present PIDcontrol method, the setting parameter values that have been set andinputted by the parameter setting device 15 are used. The settingparameter values may be different depending on values of the targetnitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas, the introduction amount of thecarbon monoxide gas and the introduction amount of the nitrogen gas as aresult of the PID control method. The gas introduction amount controller14 transmits control signals to the first supply amount controller 22for the ammonia gas, the second supply amount controller 26 for theammonia decomposition gas (whose flow rate is constant), the thirdsupply amount controller 62 for the carbon monoxide gas and the fourthsupply amount controller 72 for the nitrogen gas, in order to realizethe respective determined introduction amounts of the furnaceintroduction gases.

According to the control as described above, the in-furnace nitridingpotential can be stably controlled in the vicinity of the targetnitriding potential. Thereby, the surface hardening treatment of thework S can be performed with extremely high quality. Specifically, afeedback control is performed with a sampling rate of about severalhundred milliseconds, and the introduction amount of the ammonia gas isincreased and decreased within a range of about 3 ml (±1.5 ml), so thatthe nitriding potential can be controlled to the target nitridingpotential (0.2) with extremely high precision since a timing of about 40minutes after starting the treatment.

Explanation of Comparative Examples

As comparative examples, controls of nitriding potential were performed.In each of them, the ammonia decomposition gas was not introduced, theratio of the introduction amounts of the ammonia gas, the nitrogen gasand the carbon monoxide gas was always maintained at 50:48:2, and thetotal introduction amount thereof was changed.

Specifically, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculated the in-furnace nitridingpotential based on the inputted hydrogen concentration signal or ammoniaconcentration signal and the inputted oxygen concentration signal. Then,the gas flow rate output adjustor 30 performed the PID control method inwhich the respective gas introduction amounts of the ammonia gas, thenitrogen gas and the carbon monoxide gas were input values, thenitriding potential calculated by the in-furnace nitriding potentialcalculator 13 was an output value, and the target nitriding potential(the set nitriding potential) was a target value, More specifically, inthe present PID control method, the nitriding potential in theprocessing furnace 2 was brought close to the target nitridingpotential, by changing the total introduction amount of the ammonia gas,the nitrogen gas and the carbon monoxide gas while keeping the ratio ofthe introduction amounts of the ammonia gas, the nitrogen gas and thecarbon monoxide gas constant. However, in the above comparativeexamples, the nitriding potential could not be stably controlled.

(Comparison Between Examples 4-1 to 4-3 and Comparative Examples)

A table of the above results is shown as FIG. 16.

(Structure of Fifth Embodiment)

As shown in FIG. 17, a furnace introduction gas supplier 20″ of a fifthembodiment further includes a fifth furnace introduction gas supplier 81for carbon dioxide gas, a fifth supply amount controller 82, a fifthsupply valve 83 and a fifth flow meter 84, in addition to the furnaceintroduction gas supplier 20′ of the fourth embodiment.

The fifth furnace introduction gas supplier 81 is formed by, forexample, a tank filled with a fifth furnace introduction gas (in thisexample, carbon dioxide gas).

The fifth supply amount controller 82 is formed by a mass flowcontroller (which can finely change a flow rate within a short timeperiod), and is interposed between the fifth furnace introduction gassupplier 81 and the fifth supply valve 83. An opening degree of thefifth supply amount controller 82 changes according to the controlsignal outputted from the gas introduction amount controller 14, Inaddition, the fifth supply amount controller 82 is configured to detecta supply amount from the fifth furnace introduction gas supplier 81 tothe fifth supply valve 83, and output an information signal includingthe detected supply amount to the gas introduction amount controller 14and the recorder 6. This information signal can be used for correctionor the like of the control performed by the gas introduction amountcontroller 14.

The fifth supply valve 83 is formed by an electromagnetic valveconfigured to switch between opened and closed states according to acontrol signal outputted from the gas introduction amount controller 14,and is interposed between the fifth supply amount controller 82 and thefifth flow meter 84.

The fifth flow meter 84 is formed by, for example, a mechanical flowmeter such as a flow-type flow meter, and is interposed between thefifth supply valve 83 and the furnace introduction gas pipe 29. Thefifth flow meter 84 detects a supply amount from the fifth supply valve83 to the furnace introduction gas pipe 29, The supply amount detectedby the fifth flow meter 84 can be provided for an operator's visualconfirmation.

In the fifth embodiment, the introduction amount C1 of the carbonmonoxide gas, the introduction amount C2 of the nitrogen gas and theintroduction amount C3 of the carbon dioxide gas, which are furnaceintroduction gases except for (other than) the ammonia gas and theammonia decomposition gas, are controlled using a factor ofproportionality c1 assigned to the carbon monoxide gas, a factor ofproportionality c2 assigned to the nitrogen gas and a factor ofproportionality c3 assigned to the carbon dioxide gas, such thatC1=c1×(A+x×B). C2=c2×(A+x×B) and C3=c3×(A+x×B), wherein the introductionamount of the ammonia gas is represented by A, the introduction amountof the ammonia decomposition gas is represented by B, and apredetermined constant is represented by x.

The other structure of the fifth embodiment is substantially the same asthat of the fourth embodiment explained with reference to FIG. 15. InFIG. 17, the same parts as those of the fourth embodiment are shown bythe same reference numerals, and detailed explanation thereof isomitted.

Operation: Example 5-1

Next, a case is explained as an example 5-1, in which the surfacehardening treatment device according to the fifth embodiment is used andthe target nitriding potential is set to 1.0. In the example 5-1 aswell, a pit furnace having a size of φ 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas, the carbon monoxide gas, the nitrogen gas and thecarbon dioxide gas are introduced into the processing furnace 2 from thefurnace introduction gas supplier 20″ according to their respectiveinitial introduction amounts. In this example, the initial introductionamount of the ammonia gas was set to 13 [l/min], the initialintroduction amount of the ammonia decomposition gas was set to 19[l/min], the initial introduction amount of the carbon monoxide gas wasset to 0.45 [l/min], the initial introduction amount of the nitrogen gaswas set to 21 [l/min], the initial introduction amount of the carbondioxide gas was set to 0.9 [l/min], x=0.5 was set, c1=0.02 was set,c2=0.94 was set, and c3=0.04. These initial introduction amounts can beset and inputted by the parameter setting device 15. Furthermore, thestirring fan drive motor 9 is driven and thus the stirring fan 8 rotatesto stir the atmospheric gases in the processing furnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (1.0 in this example) and a standard margin.This standard margin can also be set and inputted by the parametersetting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (1.1 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PID control method in which therespective gas introduction amounts of the ammonia gas, the carbonmonoxide gas, the nitrogen gas and the carbon dioxide gas among the fivekinds of furnace introduction gases are input values, the nitridingpotential calculated by the in-furnace nitriding potential calculator 13is an output value, and the target nitriding potential (the setnitriding potential) is a target value. Specifically, in the present PIDcontrol method, the nitriding potential in the processing furnace 2 isbrought close to the target nitriding potential while the relationshipsof C1=c1×(A+x×B), C2=c2×(A+x×B) and C3=c3×(A+x×B) are maintained, bychanging the introduction amount of the ammonia gas, the introductionamount of the carbon monoxide gas, the introduction amount of thenitrogen gas and the introduction amount of the carbon dioxide gas whilekeeping the introduction amount of the ammonia decomposition gasconstant. In the present PID control method, the setting parametervalues that have been set and inputted by the parameter setting device15 are used. The setting parameter values may be different depending onvalues of the target nitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas, the introduction amount of thecarbon monoxide gas, the introduction amount of the nitrogen gas and theintroduction amount of the carbon dioxide gas as a result of the PIDcontrol method. The gas introduction amount controller 14 transmitscontrol signals to the first supply amount controller 22 for the ammoniagas, the second supply amount controller 26 for the ammoniadecomposition gas (whose flow rate is constant), the third supply amountcontroller 62 for the carbon monoxide gas, the fourth supply amountcontroller 72 for the nitrogen gas and the fifth supply amountcontroller 82 for the carbon dioxide gas, in order to realize therespective determined introduction amounts of the furnace introductiongases.

According to the control as described above, the in-furnace nitridingpotential can be stably controlled in the vicinity of the targetnitriding potential. Thereby, the surface hardening treatment of thework S can be performed with extremely high quality. Specifically, afeedback control is performed with a sampling rate of about severalhundred milliseconds, and the introduction amount of the ammonia gas isincreased and decreased within a range of about 3 ml (±1.5 ml), so thatthe nitriding potential can be controlled to the target nitridingpotential (1.0) with extremely high precision since a timing of about 30minutes after starting the treatment.

Operation: Example 5-2

Next, another case is explained as an example 5-2, in which the surfacehardening treatment device according to the fifth embodiment is used andthe target nitriding potential is set to 0.6. In the example 5-2 aswell, a pit furnace having a size of φ 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas, the carbon monoxide gas, the nitrogen gas and thecarbon dioxide gas are introduced into the processing furnace 2 from thefurnace introduction gas supplier 20″ according to their respectiveinitial introduction amounts. In this example, the initial introductionamount of the ammonia gas was set to 12 [l/min], the initialintroduction amount of the ammonia decomposition gas was set to 25[l/min], the initial introduction amount of the carbon monoxide gas wasset to 0.5 [l/min], the initial introduction amount of the nitrogen gaswas set to 23 [l/min], the initial introduction amount of the carbondioxide gas was set to 1.0 [l/min]. x=0.5 was set, c1=0.02 was set,c2=0.94 was set, and c3=0.04. These initial introduction amounts can beset and inputted by the parameter setting device 15. Furthermore, thestirring fan drive motor 9 is driven and thus the stirring fan 8 rotatesto stir the atmospheric gases in the processing furnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state),

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (1.0 in this example) and a standard margin.This standard margin can also be set and inputted by the parametersetting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (0.7 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PIC control method in which therespective gas introduction amounts of the ammonia gas, the carbonmonoxide gas, the nitrogen gas and the carbon dioxide gas among the fivekinds of furnace introduction gases are input values, the nitridingpotential calculated by the in-furnace nitriding potential calculator 13is an output value, and the target nitriding potential (the setnitriding potential) is a target value. Specifically, in the present PIDcontrol method, the nitriding potential in the processing furnace 2 isbrought close to the target nitriding potential while the relationshipsof C1=c1×(A+x×B), C2=c2×(A+x×B) and C3=c3×(A+x×B) are maintained, bychanging the introduction amount of the ammonia gas, the introductionamount of the carbon monoxide gas, the introduction amount of thenitrogen gas and the introduction amount of the carbon dioxide gas whilekeeping the introduction amount of the ammonia decomposition gasconstant. In the present PID control method, the setting parametervalues that have been set and inputted by the parameter setting device15 are used. The setting parameter values may be different depending onvalues of the target nitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas, the introduction amount of thecarbon monoxide gas, the introduction amount of the nitrogen gas and theintroduction amount of the carbon dioxide gas as a result of the PIDcontrol method. The gas introduction amount controller 14 transmitscontrol signals to the first supply amount controller 22 for the ammoniagas, the second supply amount controller 26 for the ammoniadecomposition gas (whose flow rate is constant), the third supply amountcontroller 62 for the carbon monoxide gas, the fourth supply amountcontroller 72 for the nitrogen gas and the fifth supply amountcontroller 82 for the carbon dioxide gas, in order to realize therespective determined introduction amounts of the furnace introductiongases.

According to the control as described above, the in-furnace nitridingpotential can be stably controlled in the vicinity of the targetnitriding potential. Thereby, the surface hardening treatment of thework S can be performed with extremely high quality. Specifically, afeedback control is performed with a sampling rate of about severalhundred milliseconds, and the introduction amount of the ammonia gas isincreased and decreased within a range of about 3 ml (±1.5 ml), so thatthe nitriding potential can be controlled to the target nitridingpotential (0.6) with extremely high precision since a timing of about 40minutes after starting the treatment.

Operation: Example 5-3

Next, another case is explained as an example 5-3, in which the surfacehardening treatment device according to the fifth embodiment is used andthe target nitriding potential is set to 0.2. In the example 5-3 aswell, a pit furnace having a size of φ 700×1000 was used as theprocessing furnace 2, 570° C. was adopted as the temperature to beheated, and a steel material having a surface area of 4 m² was used asthe work S.

While the processing furnace 2 is heated, the ammonia gas, the ammoniadecomposition gas, the carbon monoxide gas, the nitrogen gas and thecarbon dioxide gas are introduced into the processing furnace 2 from thefurnace introduction gas supplier 20″ according to their respectiveinitial introduction amounts. In this example, the initial introductionamount of the ammonia gas was set to 3 [l/min], the initial introductionamount of the ammonia decomposition gas was set to 29 [l/min], theinitial introduction amount of the carbon monoxide gas was set to 0.3[l/min], the initial introduction amount of the nitrogen gas was set to16 [l/min], the initial introduction amount of the carbon dioxide gaswas set to 0.6 [l/min], x=0.5 was set, c1=0.02 was set, c2=0.94 was set,and c3=0.04. These initial introduction amounts can be set and inputtedby the parameter setting device 15. Furthermore, the stirring fan drivemotor 9 is driven and thus the stirring fan 8 rotates to stir theatmospheric gases in the processing furnace 2.

In the initial state, the on-off valve controller 16 closes the on-offvalve 17.

In addition, the in-furnace temperature measurement device 10 measures atemperature of the in-furnace gases, and outputs an information signalincluding the measured temperature to the nitriding potential adjustor 4and the recorder 6. The nitriding potential adjustor 4 judges whetherthe state in the processing furnace 2 is still during the temperaturerising step or already after the temperature rising step has beencompleted (a stable state).

In addition, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculates an in-furnace nitridingpotential (which is initially an extremely high value (since no hydrogengas exists in the furnace), but decreases as decomposition of theammonia gas (generation of the hydrogen gas) proceeds) and judgeswhether the calculated value has dropped lower than the sum of thetarget nitriding potential (1.0 in this example) and a standard margin.This standard margin can also be set and inputted by the parametersetting device 15, and is for example 0.1.

When it is determined that the temperature rising step has beencompleted and also it is determined that the calculated value of thein-furnace nitriding potential has dropped lower than the sum (0.3 inthis example) of the target nitriding potential and the standard margin,the nitriding potential adjustor 4 starts to control an introductionamount of each of the furnace introduction gases via the gasintroduction amount controller 14. Herein, the on-off valve controller16 opens the on-off valve 17.

When the on-off valve 17 is opened, the processing furnace 2 and theatmospheric gas concentration detector 3 communicate with each other,and then the atmospheric gas concentration detector 3 detects anin-furnace hydrogen concentration or an in-furnace ammoniaconcentration, and detects an in-furnace oxygen concentration. Thedetected hydrogen concentration signal or ammonia concentration signaland the detected oxygen concentration signal are outputted to thenitriding potential adjustor 4 and the recorder 6.

The in-furnace nitriding potential calculator 13 of the nitridingpotential adjustor 4 calculates the in-furnace nitriding potential basedon the inputted hydrogen concentration signal or ammonia concentrationsignal and the inputted oxygen concentration signal. Then, the gas flowrate output adjustor 30 performs the PID control method in which therespective gas introduction amounts of the ammonia gas, the carbonmonoxide gas, the nitrogen gas and the carbon dioxide gas among the fivekinds of furnace introduction gases are input values, the nitridingpotential calculated by the in-furnace nitriding potential calculator 13is an output value, and the target nitriding potential (the setnitriding potential) is a target value. Specifically, in the present PIDcontrol method, the nitriding potential in the processing furnace 2 isbrought close to the target nitriding potential while the relationshipsof C1=c1×(A+x×B), C2=c2×(A+x×B) and C3=c3×(A+x×B) are maintained, bychanging the introduction amount of the ammonia gas, the introductionamount of the carbon monoxide gas, the introduction amount of thenitrogen gas and the introduction amount of the carbon dioxide gas whilekeeping the introduction amount of the ammonia decomposition gasconstant. In the present PID control method, the setting parametervalues that have been set and inputted by the parameter setting device15 are used. The setting parameter values may be different depending onvalues of the target nitriding potential.

Then, the gas introduction amount controller 14 controls theintroduction amount of the ammonia gas, the introduction amount of thecarbon monoxide gas, the introduction amount of the nitrogen gas and theintroduction amount of the carbon dioxide gas as a result of the PIDcontrol method. The gas introduction amount controller 14 transmitscontrol signals to the first supply amount controller 22 for the ammoniagas, the second supply amount controller 26 for the ammoniadecomposition gas (whose flow rate is constant), the third supply amountcontroller 62 for the carbon monoxide gas, the fourth supply amountcontroller 72 for the nitrogen gas and the fifth supply amountcontroller 82 for the carbon dioxide gas, in order to realize therespective determined introduction amounts of the furnace introductiongases.

According to the control as described above, the in-furnace nitridingpotential can be stably controlled in the vicinity of the targetnitriding potential. Thereby, the surface hardening treatment of thework S can be performed with extremely high quality. Specifically, afeedback control is performed with a sampling rate of about severalhundred milliseconds, and the introduction amount of the ammonia gas isincreased and decreased within a range of about 3 ml (±1.5 ml), so thatthe nitriding potential can be controlled to the target nitridingpotential (0.2) with extremely high precision since a timing of about 40minutes after starting the treatment.

Explanation of Comparative Examples

As comparative examples, controls of nitriding potential were performed.In each of them, the ammonia decomposition gas was not introduced, theratio of the introduction amounts of the ammonia gas, the nitrogen gas,the carbon monoxide gas and the carbon dioxide gas was always maintainedat 50:47:1:2, and the total introduction amount thereof was changed.

Specifically, the in-furnace nitriding potential calculator 13 of thenitriding potential adjustor 4 calculated the in-furnace nitridingpotential based on the inputted hydrogen concentration signal or ammoniaconcentration signal and the inputted oxygen concentration signal. Then,the gas flow rate output adjustor 30 performed the PID control method inwhich the respective gas introduction amounts of the ammonia gas, thenitrogen gas, the carbon monoxide gas and the carbon dioxide gas wereinput values, the nitriding potential calculated by the in-furnacenitriding potential calculator 13 was an output value, and the targetnitriding potential (the set nitriding potential) was a target value.More specifically, in the present PID control method, the nitridingpotential in the processing furnace 2 was brought close to the targetnitriding potential, by changing the total introduction amount of theammonia gas, the nitrogen gas, the carbon monoxide gas and the carbondioxide gas while keeping the ratio of the introduction amounts of theammonia gas, the nitrogen gas, the carbon monoxide gas and the carbondioxide gas constant.

However, in the above comparative examples, the nitriding potentialcould not be stably controlled.

Comparison Between Examples 5-1 to 5-3 and Comparative Examples

A table of the above results is shown as FIG. 18.

DESCRIPTION OF REFERENCE SIGNS

-   1 Surface hardening treatment device-   2 Processing furnace-   3 Atmospheric gas concentration detector-   4 Nitriding potential adjustor-   5 Temperature adjustor-   6 Recorder-   8 Stirring fan-   9 Stirring-fan drive motor-   10 In-furnace temperature measuring device-   11 Furnace body heater-   13 In-furnace nitriding potential calculator-   14 Gas introduction controller-   15 Parameter setting device (touch panel)-   16 On-off valve controller-   17 On-off valve-   20, 20′. 20″ Furnace introduction gas supplier-   21 First furnace introduction gas supplier-   22 First supply amount controller-   23 First supply valve-   24 First flow meter-   25 Second furnace introduction gas supplier-   26 Second supply amount controller-   27 Second supply valve-   28 Second flow meter-   29 Furnace introduction gas pipe-   30 Gas flow rate output adjustor-   31 Programmable logic controller-   40 Exhaust gas pipe-   41 Exhaust gas combustion decomposition apparatus-   61, 61′ Third furnace introduction gas supplier-   62 Third supply amount controller-   63 Third supply valve-   64 Third flow meter-   71 Fourth furnace introduction gas supplier-   72 Fourth supply amount controller-   73 Fourth supply valve-   74 Fourth flow meter-   81 Fifth furnace introduction gas supplier-   82 Fifth supply amount controller-   83 Fifth supply valve-   84 Fifth flow meter

1. A surface hardening treatment device for performing a gasnitrocarburizing treatment as a surface hardening treatment for a workarranged in a processing furnace by introducing a plurality of furnaceintroduction gases including an ammonia gas and an ammonia decompositiongas, the surface hardening treatment device comprising an in-furnaceatmospheric gas concentration detector configured to detect a hydrogenconcentration or an ammonia concentration in the processing furnace, anin-furnace nitriding potential calculator configured to calculate anitriding potential in the processing furnace based on the hydrogenconcentration or the ammonia concentration detected by the in-furnaceatmospheric gas concentration detector, and a gas-introduction-amountcontroller configured to change an introduction amount of each of theplurality of furnace introduction gases except for the ammoniadecomposition gas while keeping an introduction amount of the ammoniadecomposition gas constant, based on the nitriding potential in theprocessing furnace calculated by the in-furnace nitriding potentialcalculator and a target nitriding potential, such that the nitridingpotential in the processing furnace is brought close to the targetnitriding potential.
 2. The surface hardening treatment device accordingto claim 1, further comprising an in-furnace oxygen concentrationdetector configured to detect an oxygen concentration in the processingfurnace, wherein the in-furnace nitriding potential calculator isconfigured to calculate the nitriding potential in the processingfurnace based on the hydrogen concentration or the ammonia concentrationdetected by the in-furnace atmospheric gas concentration detector andthe oxygen concentration detected by the in-furnace oxygen concentrationdetector.
 3. The surface hardening treatment device according to claim 1wherein the gas-introduction-amount controller is configured to controlthe introduction amount C1, * * * , CN (N is an integer of one or more)of each of the plurality of furnace introduction gases except for theammonia gas and the ammonia decomposition gas, using a factor ofproportionality c1, * * * , cN assigned to each of the plurality offurnace introduction gases except for the ammonia gas and the ammoniadecomposition gas, such thatC1=c1×(A+x×B), * * * ,cN=cN×(A+x×B) wherein the introduction amount ofthe ammonia gas is represented by A, the introduction amount of theammonia decomposition gas is represented by B, and a predeterminedconstant is represented by x.
 4. The surface hardening treatment deviceaccording to claim 3, wherein the predetermined constant x is within 0.4to 0.6.
 5. The surface hardening treatment device according to claim 4,wherein the predetermined constant x is 0.5.
 6. The surface hardeningtreatment device according to claim 1, wherein the plurality of furnaceintroduction gases includes a carbon dioxide gas.
 7. The surfacehardening treatment device according to claim 1, wherein the pluralityof furnace introduction gases includes a carbon monoxide gas.
 8. Thesurface hardening treatment device according to claim 1, wherein theplurality of furnace introduction gases includes a carbon dioxide gasand a nitrogen gas.
 9. The surface hardening treatment device accordingto claim 1, wherein the plurality of furnace introduction gases includesa carbon monoxide gas and a nitrogen gas.
 10. A surface hardeningtreatment method of performing a gas nitrocarburizing treatment as asurface hardening treatment for a work arranged in a processing furnaceby introducing a plurality of furnace introduction gases including anammonia gas and an ammonia decomposition gas, the surface hardeningtreatment method comprising an in-furnace atmospheric gas concentrationdetecting step of detecting a hydrogen concentration or an ammoniaconcentration in the processing furnace, an in-furnace nitridingpotential calculating step of calculating a nitriding potential in theprocessing furnace based on the hydrogen concentration or the ammoniaconcentration detected at the in-furnace atmospheric gas concentrationdetecting step, and a gas-introduction-amount controlling step ofchanging an introduction amount of each of the plurality of furnaceintroduction gases except for the ammonia decomposition gas whilekeeping an introduction amount of the ammonia decomposition gasconstant, based on the nitriding potential in the processing furnacecalculated at the in-furnace nitriding potential calculating step and atarget nitriding potential, such that the nitriding potential in theprocessing furnace is brought close to the target nitriding potential.11. A surface hardening treatment device for performing a gasnitrocarburizing treatment as a surface hardening treatment for a workarranged in a processing furnace by introducing a plurality of furnaceintroduction gases including an ammonia gas, an ammonia decompositiongas and a carburizing gas, the surface hardening treatment devicecomprising an in-furnace atmospheric gas concentration detectorconfigured to detect a hydrogen concentration or an ammoniaconcentration in the processing furnace, an in-furnace nitridingpotential calculator configured to calculate a nitriding potential inthe processing furnace based on the hydrogen concentration or theammonia concentration detected by the in-furnace atmospheric gasconcentration detector, and a gas-introduction-amount controllerconfigured to change an introduction amount of each of the ammonia gasand the carburizing gas while keeping an introduction amount of theammonia decomposition gas constant, based on the nitriding potential inthe processing furnace calculated by the in-furnace nitriding potentialcalculator and a target nitriding potential, such that the nitridingpotential in the processing furnace is brought close to the targetnitriding potential.
 12. The surface hardening treatment deviceaccording to claim 11, wherein the gas-introduction-amount controller isconfigured to control the introduction amount C1 of the carburizing gas,using a factor of proportionality c1 assigned to the carburizing gas,such thatC1=c1×(A+x×B) wherein the introduction amount of the ammonia gas isrepresented by A, the introduction amount of the ammonia decompositiongas is represented by B, and a predetermined constant is represented byx.
 13. A surface hardening treatment device for performing a gasnitrocarburizing treatment as a surface hardening treatment for a workarranged in a processing furnace by introducing a plurality of furnaceintroduction gases including an ammonia gas, an ammonia decompositiongas, a carburizing gas and a nitrogen gas, the surface hardeningtreatment device comprising an in-furnace atmospheric gas concentrationdetector configured to detect a hydrogen concentration or an ammoniaconcentration in the processing furnace, an in-furnace nitridingpotential calculator configured to calculate a nitriding potential inthe processing furnace based on the hydrogen concentration or theammonia concentration detected by the in-furnace atmospheric gasconcentration detector, and a gas-introduction-amount controllerconfigured to change an introduction amount of each of the ammonia gas,the carburizing gas and the nitrogen gas while keeping an introductionamount of the ammonia decomposition gas constant, based on the nitridingpotential in the processing furnace calculated by the in-furnacenitriding potential calculator and a target nitriding potential, suchthat the nitriding potential in the processing furnace is brought closeto the target nitriding potential.
 14. The surface hardening treatmentdevice according to claim 13, wherein the gas-introduction-amountcontroller is configured to control the introduction amount C1 of thecarburizing gas and the introduction amount C2 of the nitrogen gas,using a factor of proportionality c1 assigned to the carburizing gas anda factor of proportionality c2 assigned to the nitrogen gas, such thatC1=c1×(A+x×B) and C2=c2×(A+x×B) wherein the introduction amount of theammonia gas is represented by A, the introduction amount of the ammoniadecomposition gas is represented by B, and a predetermined constant isrepresented by x.
 15. The surface hardening treatment device accordingto claim 2, wherein the gas-introduction-amount controller is configuredto control the introduction amount C1, * * * , CN (N is an integer ofone or more) of each of the plurality of furnace introduction gasesexcept for the ammonia gas and the ammonia decomposition gas, using afactor of proportionality c1, * * * , cN assigned to each of theplurality of furnace introduction gases except for the ammonia gas andthe ammonia decomposition gas, such thatC1=c1×(A+x×B), * * * ,cN=cN×(A+x×B) wherein the introduction amount ofthe ammonia gas is represented by A, the introduction amount of theammonia decomposition gas is represented by B, and a predeterminedconstant is represented by x.
 16. The surface hardening treatment deviceaccording to claim 15, wherein the predetermined constant x is within0.4 to 0.6.
 17. The surface hardening treatment device according toclaim 16, wherein the predetermined constant x is 0.5.